Power storage device and electronic device

ABSTRACT

To improve the flexibility of a power storage device, or provide a high-capacity power storage device. The power storage device includes a positive electrode, a negative electrode, an exterior body, and an electrolyte. The outer periphery of each of the positive electrode active material layer and the negative electrode active material layer is a closed curve. The exterior body includes a film and a thermocompression-bonded region. The inner periphery of the thermocompression-bonded region is a closed curve. The electrolyte, the positive electrode active material layer, and the negative electrode active material layer are in a region surrounded by the thermocompression-bonded region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a power storagedevice and an electronic device.

Note that one embodiment of the present invention is not limited to theabove technical field. One embodiment of the invention disclosed in thisspecification and the like relates to an object, a method, and amanufacturing method. One embodiment of the present invention relates toa process, a machine, manufacture, or a composition of matter.Specifically, examples of the technical field of one embodiment of thepresent invention disclosed in this specification include asemiconductor device, a display device, a light-emitting device, a powerstorage device, a memory device, a method for driving any of them, and amethod for manufacturing any of them.

Note that electronic devices in this specification generally meandevices driven by electricity; and electro-optical devices, informationterminal devices, and the like are all electronic devices. Someelectronic devices incorporate a power storage device.

Note that “incorporate” in this specification refers not only toincorporation of a power storage device in a manner that the devicecannot be detached for replacement, but also to incorporation of a powerstorage device in a manner that the device as a form of battery pack orthe like can be freely detached.

2. Description of the Related Art

In recent years, a variety of power storage devices such as lithium-ionsecondary batteries, lithium-ion capacitors, and air batteries have beenactively developed. In particular, demand for lithium-ion secondarybatteries with high output and high energy density has rapidly grownwith the development of the semiconductor industry, for the uses ofelectronic equipment, for example, portable information terminals suchas mobile phones, smartphones, and laptop computers, portable musicplayers, and digital cameras; medical equipment; and next-generationclean energy vehicles such as hybrid electric vehicles (HEVs), electricvehicles (EVs), and plug-in hybrid electric vehicles (PHEVs). Thelithium-ion secondary batteries are essential for today's informationsociety as rechargeable energy supply sources.

The development of wearable devices that are used while being worn bythe users is also actively carried out. In order to be used morecomfortably by the users, wearable devices often have curved shapes orhave flexibility. In addition, power storage devices with flexibilityand bendability to be incorporated in such wearable devices are beingdeveloped.

For example, Patent Document 1 discloses a sheet-like power storagedevice which can be bent in at least one axis direction, and electronicdevices incorporating the power storage device. Patent Document 2discloses a flexible secondary battery and an arm-worn electronic deviceincluding the secondary battery.

REFERENCES Patent Documents

[Patent Document 1] Japanese Published Patent Application No.2013-211262

[Patent Document 2] Japanese Published Patent Application No. 2015-38868

SUMMARY OF THE INVENTION

To offer wearable devices with a variety of functions and shapes, powerstorage devices need to have improved flexibility. In addition, thedevelopment of high-capacity power storage devices is required to reducethe frequency of charging wearable devices.

In view of the above, an object of one embodiment of the presentinvention is to improve the flexibility of a power storage device.Another object of one embodiment of the present invention is to providea power storage device that can be bent in any direction. Another objectof one embodiment of the present invention is to provide a high-capacitypower storage device. Another object of one embodiment of the presentinvention is to provide a highly reliable power storage device.

Furthermore, an object of one embodiment of the present invention is toprovide a flexible electronic device. Another object of one embodimentof the present invention is to provide an electronic device having acurved portion.

Furthermore, an object of one embodiment of the present invention is toprovide a novel electrode, a novel power storage device, a novelelectronic device, or the like. Note that the description of theseobjects does not exclude the existence of other objects. In oneembodiment of the present invention, there is no need to achieve all theobjects. Other objects will be apparent from and can be derived from thedescription of the specification, the drawings, the claims, and thelike.

One embodiment of the present invention is a power storage deviceincluding a positive electrode, a negative electrode, an exterior body,and an electrolyte. The positive electrode includes a positive electrodecurrent collector and a positive electrode active material layer incontact with the positive electrode current collector. The negativeelectrode includes a negative electrode current collector and a negativeelectrode active material layer in contact with the negative electrodecurrent collector. The positive electrode active material layer and thenegative electrode active material layer overlap with each other. Theouter periphery of each of the positive electrode active material layerand the negative electrode active material layer is a closed curve. Theexterior body includes a film and a thermocompression-bonded region. Theinner periphery of the thermocompression-bonded region is a closedcurve. The electrolyte, the positive electrode active material layer,and the negative electrode active material layer are in a regionsurrounded by the thermocompression-bonded region.

In the above embodiment, preferably, the outer periphery of each of thepositive electrode active material layer and the negative electrodeactive material layer is approximately circular and the inner peripheryof the thermocompression-bonded region is approximately circular.

Another embodiment of the present invention is a power storage deviceincluding a positive electrode, a negative electrode, an exterior body,and an electrolyte. The positive electrode includes a positive electrodecurrent collector and a positive electrode active material layer incontact with the positive electrode current collector. The negativeelectrode includes a negative electrode current collector and a negativeelectrode active material layer in contact with the negative electrodecurrent collector. The positive electrode active material layer and thenegative electrode active material layer overlap with each other. Theouter periphery of each of the positive electrode active material layerand the negative electrode active material layer is a closed curve. Theexterior body includes a film. The exterior body includes a firstthermocompression-bonded region and a second thermocompression-bondedregion. The first thermocompression-bonded region is surrounded by thesecond thermocompression-bonded region. The outer periphery of the firstthermocompression-bonded region is a closed curve. The inner peripheryof the second thermocompression-bonded region is a closed curve. Theexterior body includes an opening in a region surrounded by the firstthermocompression-bonded region. The electrolyte, the positive electrodeactive material layer, and the negative electrode active material layerare in a region between the first thermocompression-bonded region andthe second thermocompression-bonded region. The positive electrodecurrent collector includes a portion extending in the opening. Thenegative electrode current collector includes a portion extending in theopening.

In the above embodiment, preferably, the outer periphery of each of thepositive electrode active material layer and the negative electrodeactive material layer is approximately circular and the inner peripheryof the second thermocompression-bonded region is approximately circular.

Another embodiment of the present invention is a power storage deviceincluding a positive electrode, a negative electrode, a positiveelectrode lead, a negative electrode lead, an exterior body, and anelectrolyte. The positive electrode includes a positive electrodecurrent collector and a positive electrode active material layer incontact with the positive electrode current collector. The negativeelectrode includes a negative electrode current collector and a negativeelectrode active material layer in contact with the negative electrodecurrent collector. The positive electrode active material layer and thenegative electrode active material layer overlap with each other.

The outer periphery of each of the positive electrode active materiallayer and the negative electrode active material layer is a closedcurve. The exterior body includes a film. The exterior body includes afirst thermocompression-bonded region and a secondthermocompression-bonded region. The first thermocompression-bondedregion is surrounded by the second thermocompression-bonded region. Theouter periphery of the first thermocompression-bonded region is a closedcurve. The inner periphery of the second thermocompression-bonded regionis a closed curve. The exterior body includes an opening in a regionsurrounded by the first thermocompression-bonded region. Theelectrolyte, the positive electrode active material layer, and thenegative electrode active material layer are in a region between thefirst thermocompression-bonded region and the secondthermocompression-bonded region. The positive electrode lead iselectrically connected to the positive electrode current collector inthe region between the first thermocompression-bonded region and thesecond thermocompression-bonded region. The positive electrode leadincludes a portion extending in the opening. The negative electrode leadis electrically connected to the negative electrode current collector inthe region between the first thermocompression-bonded region and thesecond thermocompression-bonded region. The negative electrode leadincludes a portion extending in the opening.

In the above embodiment, preferably, the outer periphery of each of thepositive electrode active material layer and the negative electrodeactive material layer is approximately circular and the inner peripheryof the second thermocompression-bonded region is approximately circular.

Another embodiment of the present invention is a power storage devicewith any of the above structures, in which the film has a projection ora depression. Another embodiment of the present invention is a powerstorage device with any of the above structures, in which the inner orouter periphery of the projection or the depression has a shape similarto that of the outer periphery of the positive electrode active materiallayer or the negative electrode active material layer.

In any of the above embodiments, preferably, the power storage devicehas flexibility.

Another embodiment of the present invention is an electronic deviceincluding the power storage device with any of the above structures anda housing having flexibility. Another embodiment of the presentinvention is an electronic device including the power storage devicewith any of the above structures and a housing having a curved portion.

According to one embodiment of the present invention, the flexibility ofa power storage device can be improved. According to another embodimentof the present invention, a power storage device that can be bent in anydirection can be provided. According to another embodiment of thepresent invention, a high-capacity power storage device can be provided.According to another embodiment of the present invention, a highlyreliable power storage device can be provided.

Furthermore, according to one embodiment of the present invention, aflexible electronic device can be provided. According to anotherembodiment of the present invention, an electronic device having acurved portion can be provided.

Furthermore, a novel electrode, a novel power storage device, or a novelelectronic device can be provided. Note that the description of theseeffects does not exclude the existence of other effects. In oneembodiment of the present invention, there is no need to achieve all theeffects. Other effects will be apparent from and can be derived from thedescription of the specification, the drawings, the claims, and thelike.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1D illustrate a power storage device of one embodiment ofthe present invention;

FIGS. 2A to 2D illustrate a power storage device of one embodiment ofthe present invention;

FIGS. 3A to 3C illustrate a power storage device of one embodiment ofthe present invention;

FIGS. 4A to 4C illustrate a power storage device of one embodiment ofthe present invention;

FIGS. 5A to 5C illustrate a power storage device of one embodiment ofthe present invention;

FIGS. 6A and 6B illustrate a power storage device of one embodiment ofthe present invention;

FIG. 7 illustrates a power storage device of one embodiment of thepresent invention;

FIGS. 8A to 8C illustrate a power storage device of one embodiment ofthe present invention;

FIG. 9 illustrates a power storage device of one embodiment of thepresent invention;

FIGS. 10A to 10D illustrate a power storage device of one embodiment ofthe present invention;

FIGS. 11A to 11D illustrate a power storage device of one embodiment ofthe present invention;

FIGS. 12A to 12D illustrate a power storage device of one embodiment ofthe present invention;

FIGS. 13A to 13C illustrate a method for manufacturing a power storagedevice of one embodiment of the present invention;

FIGS. 14A and 14B illustrate a method for manufacturing a power storagedevice of one embodiment of the present invention;

FIGS. 15A and 15B illustrate a method for manufacturing a power storagedevice of one embodiment of the present invention;

FIGS. 16A to 16C illustrate a method for manufacturing a power storagedevice of one embodiment of the present invention;

FIGS. 17A to 17C illustrate a method for manufacturing a power storagedevice of one embodiment of the present invention;

FIGS. 18A and 18B illustrate a method for manufacturing a power storagedevice of one embodiment of the present invention;

FIGS. 19A to 19C illustrate a method for manufacturing a power storagedevice of one embodiment of the present invention;

FIGS. 20A to 20C illustrate a method for manufacturing a power storagedevice of one embodiment of the present invention;

FIGS. 21A to 21F illustrate a method for manufacturing a power storagedevice of one embodiment of the present invention;

FIGS. 22A and 22B illustrate active materials that can be used for apower storage device;

FIGS. 23A and 23B illustrate conductive additives and the like;

FIGS. 24A and 24B illustrate conductive additives and the like;

FIG. 25 illustrates an electronic device of one embodiment of thepresent invention;

FIGS. 26A to 26F illustrate electronic devices of one embodiment of thepresent invention;

FIGS. 27A to 27C illustrate electronic devices of one embodiment of thepresent invention;

FIGS. 28A and 28B illustrate an electronic device of one embodiment ofthe present invention;

FIGS. 29A and 29B illustrate an electronic device of one embodiment ofthe present invention;

FIG. 30 illustrates electronic devices of one embodiment of the presentinvention;

FIGS. 31A and 31B illustrate electronic devices of one embodiment of thepresent invention; and

FIGS. 32A and 32B illustrate electronic devices of one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the drawings. However, the present invention is notlimited to the description below, and it is easily understood by thoseskilled in the art that modes and details disclosed herein can bemodified in various ways. Furthermore, the present invention is notconstrued as being limited to the description of the embodiments givenbelow.

In this specification and the like, the term “connection” includesconnection between components through an “object having any electricfunction”. There is no particular limitation on the “object having anyelectric function” as long as electric signals can be transmitted andreceived between components that are connected through the object.

Note that the terms “film” and “layer” can be interchanged with eachother depending on the case or circumstances. For example, the term“conductive layer” can be changed into the term “conductive film” insome cases. Also, the term “insulating film” can be changed into theterm “insulating layer” in some cases.

The position, size, range, or the like of each component illustrated indrawings and the like is not accurately represented in some cases foreasy understanding. Therefore, the disclosed invention is notnecessarily limited to the position, size, range, or the like disclosedin the drawings and the like.

In this specification and the like, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not mean limitation of the number ofcomponents.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.In addition, the term “substantially parallel” indicates that the angleformed between two straight lines is greater than or equal to −30° andless than or equal to 30°. The term “perpendicular” indicates that theangle formed between two straight lines is greater than or equal to 80°and less than or equal to 100°, and accordingly also includes the casewhere the angle is greater than or equal to 85° and less than or equalto 95°. In addition, the term “substantially perpendicular” indicatesthat the angle formed between two straight lines is greater than orequal to 60° and less than or equal to 120°.

Embodiment 1

In this embodiment, a power storage device of one embodiment of thepresent invention will be described with reference to FIGS. 1A to 9.

1. Basic Structure

First, a power storage device 100 having a basic structure is describedwith reference to FIGS. 1A to 3C. FIG. 1A is a front view of the powerstorage device 100; FIG. 1B, a side view of the power storage device100; FIG. 1C, a front view of a positive electrode 111 included in thepower storage device 100; and FIG. 1D, a front view of a negativeelectrode 115 included in the power storage device 100.

FIGS. 2A and 2B are cross-sectional views of the power storage device100 illustrated in FIG. 1A along dashed-dotted line AB and dashed-dottedline CD, respectively. FIG. 2C is a cross-sectional view of the positiveelectrode 111 illustrated in FIG. 1C along dashed-dotted line EF. FIG.2D is a cross-sectional view of the negative electrode 115 illustratedin FIG. 1D along dashed-dotted line GH.

As illustrated in FIGS. 1A and 1B, the power storage device 100 includesan exterior body 110, and a positive electrode lead 141 and a negativeelectrode lead 145 that serve as terminal electrodes. The exterior body110 includes a thermocompression-bonded region 120, which is formed byoverlapping a circular film 109 a and a circular film 109 b and heatingthe edges of the films. Part of each of the positive and negativeelectrode leads 141 and 145 is covered with the exterior body 110. Theother part of each of the positive and negative electrode leads 141 and145 extends to the outside of the exterior body 110. In addition, asealing layer 121 made of a thermoplastic resin such as polypropylene(PP) is provided in part of each of the positive and negative electrodeleads 141 and 145 that overlaps with the thermocompression-bonded region120, thereby improving the adhesion between the films 109 a and 109 b,the adhesion between the positive electrode lead 141 and the films 109 aand 109 b, and the adhesion between the negative electrode lead 145 andthe films 109 a and 109 b.

As described above, the power storage device 100 of one embodiment ofthe present invention includes the exterior body 110 formed using thefilms 109 a and 109 b. The use of the flexible films improves theflexibility of the power storage device 100.

As the films, a metal foil laminate film, which is formed by stacking aplastic film and metal foil, can be preferably used because it can besealed by thermocompression bonding and has the advantages such as ahigh degree of shape freedom, lightweight, and flexibility. The metalfoil included in the metal foil laminate film can be formed of aluminum,stainless steel, copper, tin, nickel steel, or the like.

The plastic film stacked on the metal foil can be formed of polyethyleneterephthalate, nylon, polyethylene, PP, or the like.

Note that in this specification and the like, “laminate” refers to aprocessing method by which thin materials, such as metal foil and aplastic film, are bonded so that they are stacked.

Alternatively, the films may each be a stack of metal foil and either asingle-layer film selected from a hybrid material film including anorganic material (e.g., an organic resin or fiber) and an inorganicmaterial (e.g., ceramic), and a carbon-containing inorganic film (e.g.,a carbon film or a graphite film), or a stacked-layer film includingthese films.

As described above, the power storage device 100 includes thethermocompression-bonded region 120 on the edge of the exterior body110. The inner periphery of the thermocompression-bonded region 120 ispreferably a closed curve as illustrated in FIG. 1A, in which case thepower storage device 100 can be bent in any direction. Furtherpreferably, the inner periphery of the thermocompression-bonded region120 is circular or approximately circular. The reason for this isdescribed with reference to FIGS. 3A to 3C.

Note that in this specification and the like, a closed curve refers to acontinuous curve having no end points. Modes of the closed curve includea circle, an ellipse, a continuous shape consisting of curved portionswith different curvatures, and the like.

When force is applied externally to the power storage device 100 tochange its shape, compressive stress is applied to part of the exteriorbody 110 of the power storage device 100, and tensile stress is appliedto the other part thereof. Due to the stress, the exterior body 110 isstrained and might be partly deformed or broken.

Note that strain is the scale of change in form indicating thedisplacement of a point of an object relative to the reference (initial)length of the object.

FIGS. 3A to 3C illustrate power storage devices with a variety ofshapes, and the direction of stress applied to an exterior body wheneach power storage device is bent is indicated by an arrow 150. FIG. 3Ais a front view of the power storage device 100. FIG. 3B illustratespart of a power storage device including a thermocompression-bondedregion whose inner periphery has a polygonal shape. FIG. 3C illustratespart of a power storage device whose exterior body is formed by foldinga film.

In the case where the inner periphery of the thermocompression-bondedregion 120 has a polygonal shape as illustrated in FIG. 3B, the exteriorbody 110 includes a vertex 151. When the power storage device is bent byexternal force while stress is applied in the direction indicated by thearrow 150, the stress is likely to be concentrated on the vertex 151 andthe periphery thereof. As a result, the exterior body 110 breaks at aportion subjected to the concentrated stress, such as the vertex 151 andthe periphery thereof, which might cause danger such as leakage ofelectrolyte solution contained in the exterior body 110.

In the case where the exterior body is formed by folding a film at abent portion 153 as illustrated in FIG. 3C, the exterior body includes avertex 152 at which the inner periphery of the thermocompression-bondedregion 120 intersects the bent portion 153. When the power storagedevice is bent and stress is applied in the direction indicated by thearrow 150, the stress is likely to be concentrated on the vertex 152 andthe periphery thereof. As a result, the exterior body 110 possiblybreaks at a portion subjected to the stress, such as the vertex 152 andthe periphery thereof. In addition, the bent portion 153 breaks moreeasily than the thermocompression-bonded region 120 when being bent.Therefore, the power storage device with such a structure cannot be bentexcept in a restricted direction in order to prevent damage of the bentportion 153.

In contrast, in the power storage device 100 illustrated in FIG. 3A, theinner periphery of the thermocompression-bonded region is a closedcurve, which includes no vertex. This prevents the exterior body 110from breaking due to bending of the power storage device 100. Thus, theexterior body 110 is unlikely to break even when stress is applied inany direction as indicated by the arrows 150, resulting in improvedflexibility of the power storage device 100. Furthermore, not includinga bent portion which easily breaks due to bending, the power storagedevice 100 can be bent in any direction.

In addition, when the inner periphery of the thermocompression-bondedregion 120 is circular or approximately circular, stress concentrationcan be prevented so that the exterior body 110 can be surely preventedfrom breaking when the power storage device 100 is bent.

As illustrated in FIGS. 2A and 2B, the power storage device 100 includesthe positive electrode 111, the negative electrode 115, a separator 107,and an electrolyte solution 108 in a region that is sandwiched betweenthe films 109 a and 109 b, which constitute the external body 110, andis surrounded by the thermocompression-bonded region 120. The positiveelectrode 111 includes a positive electrode current collector 101 and apositive electrode active material layer 102 in contact with thepositive electrode current collector 101. The positive electrode lead141 is connected to the positive electrode current collector 101. Thenegative electrode 115 includes a negative electrode current collector105 and a negative electrode active material layer 106 in contact withthe negative electrode current collector 105. The negative electrodelead 145 is connected to the negative electrode current collector 105(not illustrated). Furthermore, the positive electrode active materiallayer 102 and the negative electrode active material layer 106 overlapwith each other with the separator 107 positioned therebetween.

As illustrated in FIGS. 1C and 2C, in the positive electrode 111, thepositive electrode current collector 101 includes a portion 101 a not incontact with the positive electrode active material layer 102 and aportion 101 b in contact with the positive electrode active materiallayer 102. The peripheries of the portion 101 b and the positiveelectrode active material layer 102 are closed curves. The portion 101 acan function as a tab (hereinafter also referred to as a positiveelectrode tab) that is used to electrically connect the positiveelectrode lead 141 to the positive electrode current collector 101.

As illustrated in FIGS. 1D and 2D, in the negative electrode 115, thenegative electrode current collector 105 includes a portion 105 a not incontact with the negative electrode active material layer 106 and aportion 105 b in contact with the negative electrode active materiallayer 106. The peripheries of the portion 105 b and the negativeelectrode active material layer 106 are closed curves. The portion 105 acan function as a tab (hereinafter also referred to as a negativeelectrode tab) that is used to electrically connect the negativeelectrode lead 145 to the negative electrode current collector 105.

In the power storage device 100, in the case where the positiveelectrode active material layer 102 has a region that does not overlapwith the negative electrode active material layer 106, a metal derivedfrom carrier ions and the like contained in the electrolyte solutionmight be deposited on the negative electrode active material layer 106.Thus, the width of a surface of the negative electrode active materiallayer 106 that faces the positive electrode active material layer 102 ispreferably greater than the width of a surface of the positive electrodeactive material layer 102 that faces the negative electrode activematerial layer 106 by 2% to 10%, more preferably 3% to 7% . As a result,the positive electrode active material layer 102 can surely overlap withthe negative electrode active material layer 106 with the separator 107positioned therebetween. In the case where both of the negative andpositive electrode active material layers 106 and 102 are circular, thediameter of the negative electrode active material layer 106 ispreferably greater than the diameter of the positive electrode activematerial layer 102 by 2% to 10%, more preferably 3% to 7%.

In the power storage device 100, as described above, the peripheries ofthe portion 101 b of the positive electrode current collector 101, whichis in contact with the positive electrode active material layer 102, andthe positive electrode active material layer 102 are closed curves.Also, the peripheries of the portion 105 b of the negative electrodecurrent collector 105, which is in contact with the negative electrodeactive material layer 106, and the negative electrode active materiallayer 106 are closed curves. This structure including no corner andstraight line portion, which are likely to cause damage of the exteriorbody 110, allows the power storage device 100 to be bent with littlebreakage of the exterior body 110.

Note that the peripheries of the portion 101 b in contact with thepositive electrode active material layer 102 and the positive electrodeactive material layer 102, and the peripheries of the portion 105 b incontact with the negative electrode active material layer 106 and thenegative electrode active material layer 106 are preferably circular orapproximately circular. In that case, the region sandwiched between thefilms of the external body 110 and surrounded by thethermocompression-bonded region 120 can be utilized efficiently.

Note that the power storage device 100 uses the positive electrode 111in which the positive electrode active material layer 102 is in contactwith each surface of the positive electrode current collector 101, andthe negative electrode 115 in which the negative electrode activematerial layer 106 is in contact with a surface of the negativeelectrode current collector 105. However, one embodiment of the presentinvention is not limited to this structure. The positive electrode 111may include the positive electrode active material layer 102 in contactwith a surface of the positive electrode current collector 101, or thenegative electrode 115 may include the negative electrode activematerial layer 106 in contact with each surface of the negativeelectrode current collector 105.

Either the positive electrode 111 or the negative electrode 115, or bothpreferably include the active material layer in contact with eachsurface of the current collector, in which case the capacity per unitvolume of the power storage device 100 can be increased.

Either the positive electrode 111 or the negative electrode 115, or bothalso preferably include the active material layer in contact with asurface of the current collector, in which case decreased capacity,degraded cycle performance, and the like due to bending of the powerstorage device 100 can be prevented.

For example, as illustrated in FIG. 2A, the negative electrode currentcollectors 105 are in contact with each other on a contact surface 125.These current collectors having the contact surface can slide on thecontact surface, which can relieve the stress applied to the electrodeswhen the power storage device 100 is bent. It is thus possible toprevent breakage of the current collector or the active material layerdue to bending of the power storage device 100, thereby preventingdecreased capacity, degraded cycle performance, and the like of thepower storage device 100.

Note that the number, size, stacking order, and the like of the positiveelectrode 111, the negative electrode 115, and the separator 107 used inthe power storage device 100 are not limited to those in the abovemethod. Other examples of the stack, which includes the positiveelectrode 111 provided with the positive electrode current collector 101and the positive electrode active material layer 102, the negativeelectrode 115 provided with the negative electrode current collector 105and the negative electrode active material layer 106, and the separator107, are described with reference to FIGS. 4A to 4C.

As illustrated in FIG. 4A, the stack in the power storage device 100 mayhave a tapered shape. This results in reduced effect of the corner ofthe stack on the exterior body 110, whereby the exterior body 110 isunlikely to break when the power storage device 100 is bent.Furthermore, the stack can be arranged so as to efficiently utilize thevolume of the region sandwiched between the films of the external body110 and surrounded by the thermocompression-bonded region.

The stack illustrated in FIG. 4A can be formed, for example, in thefollowing manner. The positive electrode 111, the negative electrode115, and the separator 107 are stacked; then, the stack is cut into atapered shape with a cutter, a laser cutter, or the like. Alternatively,the positive electrode 111, the negative electrode 115, and theseparator 107 are each cut with a cutter, a laser cutter, or the like tohave tapered edges;

then, they are stacked. Each edge of the positive electrode 111, thenegative electrode 115, and the separator 107 is not necessarilytapered, and the positive electrode 111, the negative electrode 115, andthe separator 107 with different sizes may be stacked to obtain atapered stack.

The stack may have a structure illustrated in FIG. 4B, which includestwo positive electrodes 111 in each of which the positive electrodeactive material layer 102 is in contact with each surface of thepositive electrode current collector 101; two negative electrodes 115 ineach of which the negative electrode active material layer 106 is incontact with a surface of the negative electrode current collector 105;and one negative electrode 115 in which the negative electrode activematerial layer 106 is in contact with each surface of the negativeelectrode current collector 105. The active material layer provided oneach surface of the current collector as illustrated in FIG. 4Bcontributes to an increase in the capacity per unit volume of the powerstorage device 100.

As in the stack illustrated in FIG. 4B, the separator 107 may have abag-like shape so that the positive electrode 111 is enclosed therein.This structure surely prevents a contact and a short-circuit between thepositive electrode 111 and the negative electrode 115.

Furthermore, as illustrated in FIG. 4C, a gel electrolyte solution 108 amay be used as the electrolyte solution 108 so that a pair of thepositive electrode 111 and the negative electrode 115, and the separator107 are bonded with the electrolyte solution 108. With this structure,the positive electrode 111 and the negative electrode 115 between whicha battery reaction occurs can be prevented from sliding when the powerstorage device 100 is bent.

In addition, many contact surfaces between metals can be obtained: acontact surface between surfaces of the positive electrodes 111 on whichthe positive electrode active material is not provided; and a contactsurface between surfaces of the negative electrodes 115 on which thenegative electrode active material is not provided. This structure ispreferable because the contact surfaces can slide to surely relieve thestress applied to the electrodes when the power storage device 100 isbent.

As a result, the power storage device 100 with little degradation, orthe highly reliable power storage device 100 can be provided.

Note that in the power storage device 100 illustrated in FIGS. 1A to 1D,the outer periphery of the thermocompression-bonded region 120 iscircular; however, one embodiment of the present invention is notlimited to this. For example, the outer periphery of thethermocompression-bonded region 120 may have a polygonal shape asillustrated in FIG. 5A, or may have a projection as illustrated in FIG.5B.

Furthermore, in the power storage device 100 illustrated in FIGS. 1A to1D and FIGS. 2A to 2D, the thermocompression-bonded region 120 has thesame width in all areas; however, one embodiment of the presentinvention is not limited to this and the thermocompression-bonded region120 may have the same width or different widths. For example, asillustrated in FIG. 5C, the thermocompression-bonded region 120 may havea larger width in a portion overlapping with the positive and negativeelectrode leads 141 and 145. This structure is preferable because thepositive and negative electrode leads 141 and 145 can be fixed to theexterior body 110 more firmly.

Modification examples of the power storage device 100 will be describedbelow. Note that for the structures, reference numerals, and drawingsthat are not specifically described in the modification examples, thedescription of Basic structure can be referred to.

2. Modification Example 1

Modification example 1 of the power storage device 100 is described withreference to FIGS. 6A and 6B. FIG. 6A is a front view of the powerstorage device 100. FIG. 6B is a cross-sectional view of the powerstorage device 100 along dashed-dotted line AB in FIG. 6A.

As illustrated in FIGS. 6A and 6B, in the power storage device 100, thefilms 109 a and 109 b of the exterior body 110 may have depressions 131and projections 132. The depressions and the projections can be formedby embossing or the like. The aforementioned materials for the films canbe easily embossed. The exterior body 110 including the depressions orthe projections, which are formed by embossing, has an increased surfacearea exposed to the air, improving the heat dissipation effect of thepower storage device 100.

In addition, the depressions or the projections on the exterior body 110contribute to suppression of breakage or the like of the exterior body110 even when the exterior body 110 is strained by force applied fromthe outside of the power storage device 100.

Furthermore, the depressions or the projections formed on the exteriorbody 110 by embossing or the like can increase the creeping distance ofthe exterior body 110 and can relieve compressive stress and tensilestress per unit length. As a result, the reliability of the powerstorage device 100 can be improved.

The depressions and projections on the exterior body 110 can thusrelieve the strain occurring when the films 109 a and 109 b receivestress due to bending of the power storage device 100. Accordingly,deformation or breakage of the films 109 a and 109 b can be prevented.

The depressions 131 or the projections 132 of the films 109 a and 109 bpreferably have a shape similar to that of the positive electrode activematerial layer 102 or the negative electrode active material layer 106,in which case strain can be relieved more easily.

More specifically, the inner periphery of the depressions 131 or theprojections 132 preferably has a shape similar to that of the outerperiphery of the positive electrode active material layer 102 or thenegative electrode active material layer 106. Alternatively, the outerperiphery of the depressions 131 or the projections 132 preferably has ashape similar to that of the outer periphery of the positive electrodeactive material layer 102 or the negative electrode active materiallayer 106.

Basic structure and Modification example 1 show an example in which thepositive electrode lead 141 and the negative electrode lead 145 areclose to each other;

however, one embodiment of the present invention is not limited to thisand as illustrated in FIG. 7, the positive electrode lead 141 and thenegative electrode lead 145 may be arranged apart from each other. Also,the positive electrode lead 141 and the negative electrode lead 145 arenot necessarily parallel to each other.

Basic structure and Modification example 1 show an example in which thepositive electrode lead 141 and the negative electrode lead 145 servingas terminal electrodes extend to the outside of the exterior body 110.Alternatively, the positive electrode lead 141 and the negativeelectrode lead 145 may extend in an opening in the exterior body 110 asshown in Modification examples 2 and 3 below.

3. Modification Example 2

Modification example 2 of the power storage device 100 is described withreference to FIGS. 8A to 8C. FIG. 8A is a front view of the powerstorage device 100. FIG. 8B is a cross-sectional view of the powerstorage device 100 along dashed-dotted line AB in FIG. 8A.

As illustrated in FIGS. 8A and 8B, the exterior body 110 may have anopening. The exterior body 110 includes two circular films 109 a and 109b each of which has a circular opening at the center and which overlapwith each other; a thermocompression-bonded region 120 a formed byheating the periphery of the openings of the films 109 a and 109 b; anda thermocompression-bonded region 120 b formed by heating the peripheryof the films 109 a and 109 b.

In the power storage device 100 shown in this modification example, theouter periphery of the thermocompression-bonded region 120 a and theinner periphery of the thermocompression-bonded region 120 b arepreferably closed curves, in which case breakage of the exterior body110 due to bending of the power storage device 100 can be prevented. Inaddition, the outer periphery of the thermocompression-bonded region 120a and the inner periphery of the thermocompression-bonded region 120 bare further preferably circular or approximately circular, in which casebreakage of the exterior body 110 can be prevented more surely.

The positive electrode 111 has an opening, and includes the positiveelectrode current collector 101 and the positive electrode activematerial layer 102 in contact with the positive electrode currentcollector 101. The negative electrode 115 has an opening, and includesthe negative electrode current collector 105 and the negative electrodeactive material layer 106 in contact with the negative electrode currentcollector 105. Furthermore, the positive electrode active material layer102 and the negative electrode active material layer 106 overlap witheach other with the separator 107 positioned therebetween. Theelectrolyte solution 108, the positive electrode active material layer102, the negative electrode active material layer 106, and the separator107 are in a region sandwiched between the films 109 a and 109 b andbetween the thermocompression-bonded regions 120 a and 120 b.

A portion of the positive electrode current collector 101 that is not incontact with the positive electrode active material layer 102 is locatednear the center of the positive electrode 111. A portion of the negativeelectrode current collector 105 that is not in contact with the negativeelectrode active material layer 106 is located near the center of thenegative electrode 115.

The portion of the positive electrode current collector 101 that is notin contact with the positive electrode active material layer 102 and theportion of the negative electrode current collector 105 that is not incontact with the negative electrode active material layer 106 are partlysurrounded by the exterior body 110, and partly extend to the outside ofthe exterior body 110.

Note that the two positive electrode current collectors 101 overlap witheach other and are bonded by ultrasonic welding to form a stack.Furthermore, the four negative electrode current collectors 105 overlapwith each other and are bonded by ultrasonic welding to form a stack.Such a structure allows terminal electrodes to be formed without use oflead electrodes.

The power storage device 100 shown in this modification example includesterminals in the center and the center is fixed by thermocompressionbonding. Accordingly, even when the power storage device is bent byexternal force, the terminal electrodes and the vicinity thereof arehardly changed in shape. Thus, damage of the terminal electrodes can bereduced, and furthermore, the number of components can be reducedbecause no lead electrode is used.

Note that when a plurality of the power storage devices 100 shown inthis modification example are stacked and the terminal electrodes of thepower storage devices 100 are connected to a wiring 156 or a wiring 157as illustrated in FIG. 8C, a power storage device with a cylindricalshape can be obtained.

Note that this structure makes it difficult to firmly attach the films109 a and 109 b to the current collectors when a larger number ofcurrent collectors are used to increase the thickness of the stack usedas a terminal electrode. Thus, a sealing layer may be provided on theportion of the positive electrode current collector 101 that is not incontact with the positive electrode active material layer 102 and theportion of the negative electrode current collector 105 that is not incontact with the negative electrode active material layer 106, therebyincreasing the adhesion between the films 109 a and 109 b, and theadhesion between the terminal electrode and each of the films.

The power storage device 100 with an opening may have a structureillustrated in FIG. 9 as shown in Modification example 3.

4. Modification Example 3

The power storage device 100 illustrated in FIG. 9 includes the exteriorbody 110 having an opening. The power storage device 100 also includesthe positive electrode lead 141, the negative electrode lead 145, andthe sealing layer 121.

Part of each of the positive and negative electrode leads 141 and 145 iscovered with the exterior body 110. The other part of each of thepositive and negative electrode leads 141 and 145 extends from theexterior body 110 to the opening in the exterior body 110. In a regioncovered with the exterior body 110, the positive electrode lead 141 isconnected to the positive electrode 111 and the negative electrode lead145 is connected to the negative electrode 115.

The sealing layer 121 provided on the positive electrode lead 141 andthe negative electrode lead 145 contributes to an increase in theadhesion between the films of the exterior body 110. Note that thepositive electrode lead 141 and the negative electrode lead 145 are notnecessarily provided with the sealing layer 121.

There is no particular limitation on the positions of the parts of thepositive electrode lead 141 and the negative electrode lead 145 thatextend in the opening in the exterior body 110; they may be close to orfar from each other.

5. Other Modification Examples

Next, other examples of the shape of the power storage device 100 willbe introduced with reference to FIGS. 10A to 12D.

As described above, preferably, the outer peripheries of the positiveand negative electrode active material layers are closed curves and theinner periphery of the thermocompression-bonded region is a closed curvein the power storage device 100. Basic structure shows an example inwhich the outer peripheries of the positive and negative electrodeactive material layers are circular and the inner periphery of thethermocompression-bonded region is circular; however, this embodiment isnot limited to this, and the outer peripheries of the positive andnegative electrode active material layers may be any other closed curve.The inner periphery of the thermocompression-bonded region may also beany other closed curve.

For example, as illustrated in FIGS. 10A to 10D, the power storagedevice 100 may include an elliptical exterior body 110, the positiveelectrode 111 including an elliptical positive electrode active materiallayer 102, the negative electrode 115 including an elliptical negativeelectrode active material layer 106, and an elliptical separator 107.Furthermore, the inner periphery of the thermocompression-bonded region120 in the exterior body 110 may have an elliptical shape.

In some cases, the outer periphery of each of the positive and negativeelectrode active material layers may have a shape with a linear portion,and the inner periphery of the thermocompression-bonded region may havea shape with a curved portion. For example, as illustrated in FIGS. 11Ato 11D, the power storage device 100 may include a semicircular exteriorbody 110, the positive electrode 111 including a semicircular positiveelectrode active material layer 102, the negative electrode 115including a semicircular negative electrode active material layer 106,and a semicircular separator 107. Furthermore, the inner periphery ofthe thermocompression-bonded region 120 may be semicircular in theexterior body 110.

Alternatively, as illustrated in FIGS. 12A to 12D, the power storagedevice 100 may include an exterior body 110 with a round-cornerquadrilateral, the positive electrode 111 including a positive electrodeactive material layer 102 with a round-corner quadrilateral, thenegative electrode 115 including a negative electrode active materiallayer 106 with a round-corner quadrilateral, and a separator 107 with around-corner quadrilateral. Furthermore, the inner periphery of thethermocompression-bonded region 120 in the exterior body 110 may be around-corner quadrilateral.

Even in the case where the positive electrode active material layer 102and the negative electrode active material layer 106 each have a shapewith a linear portion as illustrated in FIGS. 11A to 12D, the exteriorbody 110 is unlikely to be damaged when the use method or the bendingdirection of the power storage device is selected as appropriate.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 2

In this embodiment, an example of a method for manufacturing the powerstorage device 100 of one embodiment of the present invention will bedescribed with reference to FIGS. 13A to 21F. Shown as an example is amethod for manufacturing the power storage device 100 with the basicstructure (see FIGS. 1A to 2D) described in Embodiment 1.

1. Preparation of Positive Electrode and Covering with Separator

First, the positive electrode active material layer 102 is formed onboth surfaces of the positive electrode current collector 101, andprocessed into a shape of the positive electrode 111. Then, the positiveelectrode 111 is sandwiched between the two separators 107 (FIG. 13A).

Then, the outer edges of the separators 107 outside the positiveelectrode 111 are bonded to form a bag-like separator 107 (FIG. 13B).The bonding of the outer edges of the separators 107 can be performedwith the use of an adhesive or the like, by ultrasonic welding, or bythermal fusion bonding.

In this embodiment, polypropylene is used as the separators 107, and theouter edges of the separators 107 are bonded by heating. The bondingportion is shown as a region 107 a in FIG. 13B. In this manner, thepositive electrode 111 can be covered with the separators 107. Theseparators 107 are formed so as to cover the positive electrode activematerial layer 102 and does not necessarily cover the whole positiveelectrode 111.

Note that the positive electrode 111 is sandwiched between the twoseparators 107 in FIG. 13A; however, one embodiment of the presentinvention is not limited to this. For example, a single separator may befolded so as to sandwich the positive electrode 111.

The outer edges of the separators 107 may be bonded intermittently orbonded at points with regular intervals as in FIG. 13B.

Note that the shape of the separator 107 is not limited to the bag-likeshape. The separator 107 may have any shape that prevents the contactbetween the positive electrode 111 and the negative electrode 115 in thepower storage device 100, and may have, for example, a flat-plate shape.In addition, the positive electrode 111 is not necessarily sandwichedbetween the separators 107 in the case where the positive electrode 111includes the positive electrode active material layer 102 on only onesurface of the positive electrode current collector 101.

2. Preparation of Negative Electrode

Next, the negative electrode active material layer 106 is formed on thenegative electrode current collector 105, and processed into a shape ofthe negative electrode 115 (FIG. 13C).

3. Stack of Positive and Negative Electrodes

Then, the positive electrodes 111 and the negative electrodes 115 arestacked (FIG. 14A). In this embodiment, two positive electrodes 111 eachhaving the positive electrode active material layer on both surfaces ofthe positive electrode current collector, and four negative electrodes115 each having the negative electrode active material layer on onesurface of the negative electrode current collector are stacked.

The positive electrodes 111 and the negative electrodes 115 arepositioned so that the positive electrode active material layer 102 andthe negative electrode active material layer 106 overlap with each otherwith the separator 107 provided therebetween. Furthermore, the surfacesof the negative electrodes 115 that are not provided with the negativeelectrode active material layer 106 are in contact with each other.

4. Connection Between Positive and Negative Electrode Leads

Next, the positive electrode lead 141 including the sealing layer 121 iselectrically connected to positive electrode tabs of the plurality ofpositive electrode current collectors 101 by ultrasonic wave irradiationwhile pressure is applied (ultrasonic welding).

The lead electrode is likely to be cracked or cut by stress due toexternal force applied after the fabrication of the power storagedevice. Thus, when subjected to ultrasonic welding, the positiveelectrode lead 141 is placed between bonding dies provided withprojections, whereby a connection region and a curved portion can beformed in the positive electrode tab. This curved portion can relievethe stress caused by external force applied after the fabrication of thepower storage device 100, thereby improving the reliability of the powerstorage device 100.

Other than the formation of the curved portion in the positive electrodetab, the following may be employed: the positive electrode currentcollector is formed using a high-strength material such as stainlesssteel or titanium to a thickness of less than or equal to 10 μm, so thatstress due to external force that is applied after the fabrication ofthe power storage device can be easily relieved.

It is needless to say that two or more of the above examples may becombined to alleviate the concentration of stress in the positiveelectrode tab.

As in the case of the positive electrode current collector 101, negativeelectrode tabs of a plurality of negative electrode current collectors105 and the negative electrode lead 145 provided with the sealing layer121 are electrically connected to each other by ultrasonic welding (FIG.14B). At this time, structures which can easily relieve stress can beemployed as in the case of the positive electrode tabs; for example, thenegative electrode tab is provided with a curved portion or the currentcollector is formed using a high-strength material.

5. Bonding of Part of Exterior Body

Next, the positive electrode 111, the positive electrode lead 141, thenegative electrode 115, and the negative electrode lead 145 aresandwiched between the films 109 a and 109 b. Then, parts of the films109 a and 109 b (a thermocompression-bonded region 122 a in FIG. 15A)are bonded to each other (FIG. 15A). The bonding can be performed bythermal welding, for example. Note that the films 109 a and 109 b mayhave depressions and projections; however, in FIGS. 15A to 16C, thedepressions and projections of the film 109 a and the exterior body 110are not illustrated for simplification.

6. Bonding of Other Part of Exterior Body and Injection of ElectrolyteSolution

Then, the electrolyte solution 108 is injected to a region sandwichedbetween the films 109 a and 109 b from a portion where the films 109 aand 109 b are not bonded (FIG. 15B).

7A. Sealing

Next, the parts of the films 109 a and 109 b that are not yet bonded (athermocompression-bonded region 122 b in FIG. 16A) are bonded by heatingand pressing under vacuum, thereby providing the bag-like exterior body110 including the films 109 a and 109 b (FIG. 16A). This treatment isperformed in an environment from which oxygen and water are eliminated,for example, in a glove box. The evacuation to a vacuum is preferablyperformed with a vacuum sealer, a liquid pouring sealer, or the like. Animpulse sealer, a heat sealer, or the like can be used for the sealing.In the case of using an impulse sealer, for example, heating may beperformed at a temperature of 175° C. and a degree of vacuum of 60 kPafor 3 seconds. In the case where a heat sealer is used and evacuation toa vacuum is not performed, heating may be performed at a temperature of165° C. and a pressure of 0.3 MPa for 4 seconds. At this time, pressuremay be applied to the positive electrode and the negative electrode fromabove the exterior body 110. The application of pressure enables removalof bubbles that enter between the positive electrode and the negativeelectrode when the electrolyte solution is injected.

8A. Aging

Next, charging and discharging are preferably performed for agingtreatment. In this specification and the like, the aging treatmentrefers to a step performed to detect an initial defect of a powerstorage device and to form a stable film on a negative electrode activematerial in initial charging and discharging. Specifically, the agingtreatment refers to steps of keeping a charging state for a long time,performing one or more cycles of charging and discharging, or the likeat a temperature close to the upper limit of the operating temperaturerange of the battery. Moreover, the aging treatment may include a stepof releasing gas generated in a region covered with the exterior body110.

When a stable film is formed on the negative electrode active materialin initial charging and discharging, consumption of carrier ions causedby further film formation in subsequent charging and discharging can beinhibited. Thus, the aging treatment allows the performance of the powerstorage device to be more stabilized and a defective cell to bedetected.

In this embodiment, after one or more cycles of charging anddischarging, part of the exterior body 110 is cut out and gas isreleased as illustrated in FIG. 16B.

9A. Resealing

Then, a side of the exterior body 110 that has been cut out in the agingtreatment (a thermocompression-bonded region 122 c in FIG. 16C) isresealed (FIG. 16C). Through the above process, the power storage device100 can be fabricated.

Described next is another method of sealing, aging, and resealing, whichis performed after the bonding of the other part of the exterior bodyand injection of an electrolyte solution.

7B. Sealing

Next, the parts of the films 109 a and 109 b that are not yet bonded (athermocompression-bonded region 122 d in FIG. 17A) are bonded by heatingand pressing under vacuum, thereby providing the bag-like exterior body110 including the films 109 a and 109 b (FIG. 17A). At this time, thetwo films are bonded with a strength so low as to allow the films to bedetached by external force. For example, the thermocompression-bondedregion 122 d is preferably formed by bonding the films at a temperaturelower than that described in 7A. In the case of using an impulse sealer,for example, heating may be performed at a temperature of 130° C. to140° C. and a degree of vacuum of 60 kPa for 3 seconds. In the casewhere a heat sealer is used and evacuation to a vacuum is not performed,heating may be performed at a temperature of 130° C. to 140° C. and apressure of 0.3 MPa for 4 seconds.

8B. Aging

Then, aging is performed in a manner similar to that described in 8A.

In this embodiment, as aging treatment, one or more cycles of chargingand discharging are performed; then, as illustrated in FIG. 17B, the twofilms are detached from each other in the thermocompression-bondedregion 122 d by force so that gas is released.

9B. Resealing

Next, the detached region is resealed (a thermocompression-bonded region122 e in FIG. 17C). This method enables the fabrication of the powerstorage device 100 including a thermocompression-bonded region with acircular inner periphery.

Another example of the method of stacking positive and negativeelectrodes described in [3. Stack of positive and negative electrodes]will be described with reference to FIGS. 18A to 20C.

FIGS. 18A and 18B illustrate a method of stacking electrodes 170 withuse of the separator 107 (see FIG. 18A) including a plurality ofconnected circles. As illustrated in FIG. 18B, the separator 107 isaccordion-folded so that the electrodes 170 are positioned betweenfolded sheets of the separator 107, whereby the electrodes 170 can bestacked.

Note that the electrode 170 refers to the positive electrode 111 or thenegative electrode 115. For clarity of the drawings, the positiveelectrode 111 and the negative electrode 115 are denoted as theelectrodes 170 in FIGS. 18A to 19C.

FIGS. 19A to 19C illustrate a method of stacking the electrodes 170 withuse of the separator 107 (see FIG. 19A) including a plurality ofconnected circles to form a stack 175 (FIG. 19B). The stack 175 includesthe electrodes 170, the separator 107, and lead electrodes. FIG. 19C isa cross-sectional view of the stack 175 along dashed dotted line AB inFIG. 19B. As illustrated in FIGS. 19A to 19C, the electrodes 170 canalso be stacked by a method of winding the separator 107 including aplurality of connected circles so that the electrodes 170 are positionedbetween folded sheet of the separator 107.

The stack 175 may be a combination of a plurality of stacks. The stack175 illustrated in FIGS. 20A to 20C includes a plurality of first stacks171 and a plurality of second stacks 172. FIGS. 20A and 20B arecross-sectional views of the first stack 171 and the second stack 172,respectively. FIG. 20C shows another example of the cross-sectional viewof the stack 175 along dashed-dotted line AB in FIG. 19B. Note that forclarity of the drawings, FIG. 20C only illustrates the first stacks 171,the second stacks 172, and a separator 107B.

As illustrated in FIG. 20A, in the first stack 171, the positiveelectrode 111 including the positive electrode active material layer 102in contact with each surface of the positive electrode current collector101, a separator 107A, the negative electrode 115 including the negativeelectrode active material layer 106 in contact with each surface of thenegative electrode current collector 105, the separator 107A, and thepositive electrode 111 including the positive electrode active materiallayer 102 in contact with each surface of the positive electrode currentcollector 101 are stacked in this order. In the second stack 172, asillustrated in FIG. 20B, the negative electrode 115 including thenegative electrode active material layer 106 in contact with eachsurface of the negative electrode current collector 105, the separator107A, the positive electrode 111 including the positive electrode activematerial layer 102 in contact with each surface of the positiveelectrode current collector 101, the separator 107A, and the negativeelectrode 115 including the negative electrode active material layer 106in contact with each surface of the negative electrode current collector105 are stacked in this order.

As illustrated in FIG. 20C, the plurality of first stacks 171 and theplurality of second stacks 172 are covered with the winding separator107B. In other words, the plurality of first stacks 171 and theplurality of second stacks 172 are positioned between winding sheet ofthe separator 107B.

Note that in the positive electrode 111 of the outermost first stack171, the positive electrode active material layer 102 is preferably incontact with only a surface of the positive electrode current collector101.

FIGS. 20A and 20B illustrate the structure in which each of the firststack and the second stack includes the three electrodes and the twoseparators; however, one embodiment of the present invention is notlimited to this. Each stack may include four or more electrodes andthree or more separators. A larger number of electrodes lead to highercapacity of the power storage device 100. Alternatively, each stack mayinclude two electrodes and one separator. A smaller number of electrodesenable the power storage device to be curved more easily. FIG. 20Cillustrates the structure in which the power storage device 100 includesthe three first stacks 171 and the two second stacks 172; however, oneembodiment of the present invention is not limited to this, and thenumber of the stacks may be increased. A larger number of stacks lead tohigher capacity of the power storage device 100. Alternatively, thenumber of the stacks may be decreased. A smaller number of stacks enablethe power storage device to be curved more easily.

Next, a method of making depressions and projections on a film used forthe exterior body 110 will be described with reference to FIGS. 21A to21F.

FIG. 21A illustrates a mold 161 and a mold 162, which are used formaking depressions and projections on a film. The molds 161 and 162 eachinclude a plurality of circular projections 160. When the film 109 a isinterposed between the molds 161 and 162 (see FIGS. 21B and 21C),circular depressions and projections can be made on the film 109 a.

FIGS. 21D to 21F are cross-sectional views of the film interposedbetween the molds. The projections of the mold 161 and the projectionsof the mold 162 may be designed so as to engage with each other asillustrated in FIG. 21D. Alternatively, the projections of the mold 161and the projections of the mold 162 may overlap with each other asillustrated in FIG. 21E. Further alternatively, the film 109 a may beinterposed between a flat plate 163 and the mold 161 as illustrated inFIG. 21F, so that depressions and projections can be made on the film109 a.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 3

In this embodiment, materials that can be used in a power storage deviceof one embodiment of the present invention will be described in detailwith reference to FIGS. 22A to 24B.

1. Positive Electrode

The positive electrode 111 includes the positive electrode currentcollector 101, the positive electrode active material layer 102 incontact with the positive electrode current collector 101, and the like.

The positive electrode current collector 101 can be formed using amaterial that has high conductivity and is not eluted with the potentialof the positive electrode, such as a metal like stainless steel, gold,platinum, aluminum, or titanium, or an alloy thereof.

Alternatively, an aluminum alloy to which an element that improves heatresistance, such as silicon, titanium, neodymium, scandium, ormolybdenum, is added can be used. Still alternatively, a metal elementthat forms silicide by reacting with silicon can be used. Examples ofthe metal element that forms silicide by reacting with silicon includezirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium,molybdenum, tungsten, cobalt, and nickel. The positive electrode currentcollector 101 can have a foil-like shape, a plate-like shape (asheet-like shape), a net-like shape, a punching-metal shape, anexpanded-metal shape, or the like as appropriate. The positive electrodecurrent collector 101 preferably has a thickness greater than or equalto 5 μm and less than or equal to 30 μm. The surface of the positiveelectrode current collector 101 may be provided with an undercoat layerusing graphite or the like.

The positive electrode active material layer 102 may further include, inaddition to the positive electrode active material, a binder forincreasing the adhesion of the positive electrode active material, aconductive additive for increasing the conductivity of the positiveelectrode active material layer 102, and the like.

Examples of the positive electrode active material that can be used forthe positive electrode active material layer 102 include a compositeoxide with an olivine crystal structure, a composite oxide with alayered rock-salt crystal structure, and a composite oxide with a spinelcrystal structure. For example, a compound such as LiFeO₂, LiCoO₂,LiNiO₂, LiMn₂O₄, V₂O₅, Cr₂O₅, or MnO₂ can be used.

In particular, LiCoO₂ is preferable because it has high capacity andhigher stability in the air and higher thermal stability than LiNiO₂,for example.

It is preferable to add a small amount of lithium nickel oxide (LiNiO₂or LiNi_(1-x)M_(x)O₂ (0<x<1) (M=Co, Al, or the like)) to alithium-containing material with a spinel crystal structure whichcontains manganese such as LiMn₂O₄ because the characteristics of thepower storage device using such a material can be improved.

In addition, a lithium-manganese composite oxide that is represented bythe composition formula Li_(a)Mn_(b)M_(c)O_(d) can be used as thepositive electrode active material. Here, the element M is preferablysilicon, phosphorus, or a metal element other than lithium andmanganese, and further preferably nickel. Furthermore, in the case wherethe whole particle of the lithium-manganese composite oxide is measured,it is preferable to satisfy the following at the time of discharging:0<a/(b+c)<2; c>0; and 0.26≦(b+c)/d<0.5. Note that the composition ratioof the metal, silicon, phosphorus, or the like in the whole particle ofthe lithium-manganese composite oxide can be measured with aninductively coupled plasma mass spectrometer (ICP-MS), for example. Thecomposition ratio of oxygen in the whole particle of thelithium-manganese composite oxide can be measured with an energydispersive X-ray spectrometer (EDX), for example, or can be obtainedusing fusion gas analysis or valence evaluation of X-ray absorption finestructure (XAFS) analysis together with ICP-MS analysis. Note that thelithium-manganese composite oxide is an oxide containing at leastlithium and manganese, and may contain at least one selected fromchromium, cobalt, aluminum, nickel, iron, magnesium, molybdenum, zinc,indium, gallium, copper, titanium, niobium, silicon, phosphorus, and thelike.

To achieve high capacity, the lithium-manganese composite oxidepreferably includes a region where the surface portion and the middleportion are different in the crystal structure, the crystal orientation,or the oxygen content. In order that such a lithium-manganese compositeoxide can be obtained, the composition formula is preferablyLi_(a)Mn_(b)Ni_(c)O_(d) satisfying the following: 1.6≦a≦1.848;0.19≦c/b≦0.935; and 2.5≦d≦3. Furthermore, it is particularly preferableto use a lithium-manganese composite oxide represented by thecomposition formula Li_(1.68)Mn_(0.8062)Ni_(0.318)O₃. In thisspecification and the like, a lithium-manganese composite oxiderepresented by the composition formula Li_(1.68)Mn_(0.8062)Ni_(0.318)O₃refers to that formed at a ratio (molar ratio) of the amounts of rawmaterials of Li₂CO₃: MnCO₃: NiO=0.84:0.8062:0.318. Although thislithium-manganese composite oxide is represented by the compositionformula Li_(1.68)Mn_(0.8062)N_(0.318)O₃, the composition might deviatefrom this.

FIGS. 22A and 22B each illustrate an example of a cross section of aparticle of the lithium-manganese composite oxide including regionswhich are different in crystal structure, crystal orientation, or oxygencontent.

As illustrated in FIG. 22A, the lithium-manganese composite oxidepreferably includes a first region 331, a second region 332, and a thirdregion 333 as the regions which are different in crystal structure,crystal orientation, or oxygen content. The second region 332 is incontact with at least part of the outside of the first region 331. Here,the term “outside” refers to the side closer to a surface of a particle.The third region 333 preferably includes a region corresponding to thesurface of the lithium-manganese composite oxide particle.

As shown in FIG. 22B, the first region 331 may include a region notcovered with the second region 332. The second region 332 may include aregion not covered with the third region 333. For example, the firstregion 331 may include a region in contact with the third region 333.The first region 331 may include a region covered with neither thesecond region 332 nor the third region 333.

The composition of the second region 332 is preferably different fromthat of the first region 331.

For example, description is made on the case where the composition ofthe first region 331 and that of the second region 332 are separatelymeasured and the first region 331 and the second region 332 each containlithium, manganese, the element M, and oxygen; the atomic ratio oflithium to manganese, the element M, and oxygen in the first region 331is represented by a1:b1:c1:d1; and the atomic ratio of lithium tomanganese, the element M, and oxygen in the second region 332 isrepresented by a2:b2:c2:d2. Note that the composition of each of thefirst region 331 and the second region 332 can be measured by, forexample, energy dispersive X-ray spectroscopy (EDX) using a transmissionelectron microscope (TEM). In measurement by EDX, the ratio of lithiumto the total composition is sometimes difficult to measure. Thus, adifference between the composition of the elements other than lithium inthe first region 331 and that in the second region 332 is describedbelow. Here, d1/(b1+c1) is preferably greater than or equal to 2.2, morepreferably greater than or equal to 2.3, and still more preferablygreater than or equal to 2.35 and less than or equal to 3. Furthermore,d2/(b2+c2) is preferably less than 2.2, more preferably less than 2.1,and still more preferably greater than or equal to 1.1 and less than orequal to 1.9. Also in this case, the composition of the whole particleof the lithium-manganese composite oxide including the first region 331and the second region 332 preferably satisfies 0.26 (b+c)/d<0.5 asdescribed above.

The valence of manganese in the second region 332 may be different fromthat of manganese in the first region 331. The valence of the element Min the second region 332 may be different from that of the element M inthe first region 331.

Specifically, the first region 331 is preferably a lithium-manganesecomposite oxide having a layered rock-salt crystal structure. The secondregion 332 is preferably a lithium-manganese composite oxide having aspinel crystal structure.

Here, in the case where there is a spatial distribution of thecomposition or the valence of an element in any of the regions, thecompositions or the valences in a plurality of portions in the regionare obtained, and the average value thereof is calculated to be regardedas the composition or the valence in the region, for example.

A transition layer may be provided between the second region 332 and thefirst region 331. Here, the transition layer is a region wherecomposition is changed continuously or gradually, a region where acrystal structure is changed continuously or gradually, or a regionwhere the lattice constant of a crystal is changed continuously orgradually. A mixed layer may be provided between the second region 332and the first region 331. The mixed layer is a region in which, forexample, two or more crystals having different crystal orientations aremixed, two or more crystals having different crystal structures aremixed, or two or more crystals having different compositions are mixed.

For the third region 333, carbon or a metal compound can be used.Examples of the metal include cobalt, aluminum, nickel, iron, manganese,titanium, zinc, and lithium. An example of the metal compound include anoxide or a fluoride of the metal.

In particular, the third region 333 preferably contains carbon among theabove. Since carbon has high conductivity, a particle coated with carbonin an electrode of the power storage device can reduce the resistance ofthe electrode, for example. When the third region 333 contains carbon,the second region 332 in contact with the third region 333 can beoxidized. The third region 333 may contain graphene, graphene oxide, orgraphene oxide subjected to reduction. Graphene and reduced grapheneoxide have excellent electrical characteristics of high conductivity andexcellent physical properties of high flexibility and high mechanicalstrength. Moreover, the particle of the lithium-manganese compositeoxide can be coated efficiently.

When the third region 333 contains carbon such as graphene, the powerstorage device using the lithium-manganese composite oxide for itspositive electrode material can have improved cycle characteristics.

The thickness of a layer containing carbon is preferably greater than orequal to 0.4 nm and less than or equal to 40 nm.

Furthermore, the average diameter of primary particles of thelithium-manganese composite oxide is preferably greater than or equal to5 nm and less than or equal to 50 μm, more preferably greater than orequal to 100 nm and less than or equal to 500 nm, for example.Furthermore, the specific surface area is preferably greater than orequal to 5 m²/g and less than or equal to 15 m²/g. Furthermore, theaverage diameter of secondary particles is preferably greater than orequal to 5 μm and less than or equal to 50 μm. Note that the averageparticle diameters can be measured with a particle size distributionanalyzer or the like using a laser diffraction and scattering method orby observation with a scanning electron microscope (SEM) or a TEM. Thespecific surface area can be measured by a gas adsorption method.

Alternatively, a complex material (LiMPO₄ (general formula) (M is one ormore of Fe(II), Mn(II), Co(II), and Ni(II))) can be used as the positiveelectrode active material. Typical examples of the general formulaLiMPO₄ include lithium compounds such as LiFePO₄, LiNiPO₄, LiCoPO₄,LiMnPO₄, LiFe_(a)Ni_(b)PO₄, LiFe_(a)Co_(b)PO₄, LiFe_(a)Mn_(b)PO₄,LiNi_(a)Co_(b)PO₄, LiNi_(a)Mn_(b)PO₄ (a+b≦1, 0<a<1, and 0<b<1),LiFe_(c)Ni_(d)Co_(e)PO₄, LiFe_(c)Ni_(d)Mn_(e)PO₄,LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≦1, 0<c<1, 0<d<1, and 0<e<1), andLiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≦1, 0<f<1, 0<g<1, 0<h<1, and0<i<1).

LiFePO₄ is particularly preferable because it properly has propertiesnecessary for the positive electrode active material, such as safety,stability, high capacity density, and the existence of lithium ions thatcan be extracted in initial oxidation (charging).

Alternatively, a complex material such as Li_((2−j))MSiO₄ (generalformula) (M is one or more of Fe(II), Mn(II), Co(II), and Ni(II); 0≦j≦2)can be used as the positive electrode active material. Typical examplesof Li_((2−j))MSiO₄ (general formula) include lithium compounds such asLip_((2−j))FeSiO₄, Li_((2−j))NiSiO₄, Li_((2−j))CoSiO₄,Lip_((2−j))MnSiO₄, Li_((2−j))Fe_(k)Ni_(l)SiO₄,Li_((2−j))Fe_(k)Co_(l)SiO₄, Li_((2−j))Fe_(k)Mn_(l)SiO₄,Li_((2−j))Ni_(k)Co_(l)SiO₄, Li_((2−j))Ni_(k)Mn_(l)SiO₄ (k+l≦1, 0<k<1,and 0<l<1), Li_((2−j))Fe_(m)Ni_(n)Co_(q)SiO₄,Li_((2−j))Fe_(m)Ni_(n)Mn_(q)SiO₄, Li_((2−j))Ni_(m)Co_(n)Mn_(q)SiO₄(m+n+q≦1, 0<m<1, 0<n<1, and 0<q<1), andLi_((2−j))Fe_(r)Ni_(s)Co_(t)Mn_(u)SiO₄ (r+s+t+u≦1, 0<r<1, 0<s<1, 0<t<1,and 0<u<1).

Still alternatively, a NASICON compound represented by a generalformula, A_(x)M₂(XO₄)₃ (A=Li, Na, or Mg, M=Fe, Mn, Ti, V, or Nb, andX=S, P, Mo, W, As, or Si), can be used as the positive electrode activematerial. Examples of the NASICON compound include Fe₂(MnO₄)₃,Fe₂(SO₄)₃, and Li₃Fe₂(PO₄)₃. Still further alternatively, a compoundrepresented by a general formula, Li₂MPO₄F, Li₂MP₂O₇, or Li₅MO₄ (M=Fe orMn), a perovskite fluoride such as NaFeF₃ or FeF₃, a metal chalcogenide(a sulfide, a selenide, or a telluride) such as TiS₂ or MoS₂, an oxidewith an inverse spinel crystal structure such as LiMVO₄, a vanadiumoxide (e.g., V₂O₅, V₆O₁₃, or LiV₃O₈), a manganese oxide, or an organicsulfur compound can be used as the positive electrode active material,for example.

In the case where carrier ions are alkali metal ions other than lithiumions or alkaline-earth metal ions, the positive electrode activematerial may contain, instead of lithium, an alkali metal (e.g., sodiumor potassium) or an alkaline-earth metal (e.g., calcium, strontium,barium, beryllium, or magnesium). For example, the positive electrodeactive material may be a layered oxide containing sodium such as NaFeO₂or

Note that although not shown, a conductive material such as a carbonlayer may be provided on a surface of the positive electrode activematerial layer 102. With the conductive material such as the carbonlayer, the conductivity of the electrode can be increased. For example,the positive electrode active material layer 102 can be coated with thecarbon layer by mixing a carbohydrate such as glucose at the time ofbaking the positive electrode active material.

The average particle diameter of the primary particle of the positiveelectrode active material is preferably greater than or equal to 50 nmand less than or equal to 100

Examples of the conductive additive include a carbon material, a metalmaterial, and a conductive ceramic material. Alternatively, a fibermaterial may be used as the conductive additive. The content of theconductive additive in the active material layer is preferably greaterthan or equal to 1 wt % and less than or equal to 10 wt %, morepreferably greater than or equal to 1 wt % and less than or equal to 5wt %.

A network for electrical conduction can be formed in the electrode bythe conductive additive. The conductive additive also allows maintainingof a path for electric conduction between the particles of the positiveelectrode active material. The addition of the conductive additive tothe active material layer increases the electrical conductivity of theactive material layer.

Examples of the conductive additive include natural graphite, artificialgraphite such as mesocarbon microbeads, and carbon fiber. Examples ofcarbon fiber include mesophase pitch-based carbon fiber, isotropicpitch-based carbon fiber, carbon nanofiber, and carbon nanotube. Carbonnanotube can be formed by, for example, a vapor deposition method. Otherexamples of the conductive additive include carbon materials such ascarbon black (e.g., acetylene black (AB)), graphite (black lead)particles, graphene, and fullerene. Alternatively, metal powder or metalfibers of copper, nickel, aluminum, silver, gold, or the like, aconductive ceramic material, or the like can be used.

Flaky graphene has an excellent electrical characteristic of highconductivity and excellent physical properties of high flexibility andhigh mechanical strength.

Thus, the use of graphene as the conductive additive can increase thecontact points and contact area of active materials.

Note that graphene in this specification includes single-layer grapheneand multilayer graphene including two to hundred layers. Single-layergraphene refers to a one-atom-thick sheet of carbon molecules having πbonds. Graphene oxide refers to a compound formed by oxidation of suchgraphene. When graphene oxide is reduced to form graphene, oxygencontained in the graphene oxide is not entirely released and part of theoxygen remains in the graphene. In the case where graphene containsoxygen, the proportion of oxygen in the graphene measured by XPS ishigher than or equal to 2 atomic % and lower than or equal to 11 atomic%, preferably higher than or equal to 3 atomic % and lower than or equalto 10 atomic %.

Graphene is capable of making low-resistance surface contact and hasextremely high conductivity even with a small thickness. Therefore, evena small amount of graphene can efficiently form a conductive path in anactive material layer.

In the case where an active material with a small average particlediameter (e.g., 1 μm or less) is used, the specific surface area of theactive material is large and thus more conductive paths for the activematerial particles are needed. In such a case, it is particularlypreferable to use graphene, which has extremely high conductivity andcan efficiently form a conductive path even in a small amount.

A cross-sectional structure example of a positive electrode activematerial layer containing graphene as a conductive additive is describedbelow. Note that a negative electrode active material layer may containgraphene as a conductive additive.

FIG. 23A is a longitudinal sectional view of the positive electrodeactive material layer 102 and the positive electrode current collector101. The positive electrode active material layer 102 includes positiveelectrode active material particles 322, graphene flakes 321 as aconductive additive, and a binder (not illustrated).

In the longitudinal section of the positive electrode active materiallayer 102, as illustrated in FIG. 23A, the sheet-like graphene flakes321 in the positive electrode active material layer 102 substantiallyuniformly cover the positive electrode active materials such thatsurface contact is made. The graphene flakes 321 are schematically shownby thick lines in FIG. 23A but are actually thin films each having athickness corresponding to the thickness of a single layer or amulti-layer of carbon molecules. The plurality of graphene flakes 321are formed in such a way as to wrap or cover the plurality of positiveelectrode active material particles 322, or adhere to the surfaces ofthe plurality of positive electrode active material particles 322, sothat the graphene flakes 321 make surface contact with the positiveelectrode active material particles 322. Furthermore, the grapheneflakes 321 are also in surface contact with each other; consequently,the plurality of graphene flakes 321 form a three-dimensional networkfor electric conduction.

This is because graphene oxide with extremely high dispersibility in apolar solvent is used for the formation of the graphene flakes 321. Thesolvent is removed by volatilization from a dispersion medium in whichgraphene oxide is uniformly dispersed, and the graphene oxide is reducedto graphene; hence, the graphene flakes 321 remaining in the positiveelectrode active material layer 102 partly overlap with each other andcover the positive electrode active material such that surface contactis made, thereby forming an electrical conduction path. Note that,graphene oxide may be reduced by, for example, heat treatment or withthe use of a reducing agent.

Unlike conductive additive particles that make point contact with anactive material, such as acetylene black, the graphene flake 321 iscapable of making low-resistance surface contact; accordingly, theelectrical conduction between the positive electrode active materialparticles 322 and the graphene flakes 321 can be improved without anincrease in the amount of a conductive additive. Thus, the proportion ofthe positive electrode active material particles 322 in the positiveelectrode active material layer 102 can be increased. Accordingly, thedischarge capacity of a power storage device can be increased.

In addition, graphene flakes are bonded to each other to form net-likegraphene (hereinafter referred to as a graphene net). The graphene netcovering the active material can function as a binder for bindingparticles. The amount of a binder can thus be reduced, or the binderdoes not have to be used. This can increase the proportion of the activematerial in the electrode volume or weight. That is to say, the capacityof the power storage device can be increased.

The aforementioned structure where the positive electrode activematerial layer or the negative electrode active material layer containsgraphene as a conductive additive is particularly effective for aflexible power storage device.

FIG. 24A is a longitudinal sectional view of the positive electrodeactive material layer 102 and the positive electrode current collector101 of a conventional example in which conductive additive particles 323such as acetylene black are used.

The positive electrode active material particles 322 are in contact withthe conductive additive particles 323, so that a network for electricalconduction is formed between the positive electrode active materialparticles 322.

FIG. 24B shows the case where the positive electrode active materiallayer 102 and the positive electrode current collector 101 in FIG. 24Aare curved. As illustrated in FIG. 24B, when the conductive additiveparticles 323 are used as a conductive additive, the distance betweenthe positive electrode active material particles 322 changes withcurving of the positive electrode active material layer 102, which mightbreak part of the network for electrical conduction between the positiveelectrode active material particles 322.

FIG. 23B shows the case where the positive electrode active materiallayer 102 containing graphene as a conductive additive, and the positiveelectrode current collector 101 in FIG. 23A are curved. Since grapheneis a flexible sheet, the network for electrical conduction can bemaintained even when the distance between the positive electrode activematerial particles 322 changes with curving of the positive electrodeactive material layer 102 as in FIG. 23B.

Electrodes used for the power storage device of one embodiment of thepresent invention can be fabricated by various methods. For example, inthe case where an active material layer is formed over a currentcollector by a coating method, the active material, the binder, theconductive additive, and the dispersion medium (also referred to as asolvent) are mixed to form a paste, the paste is applied to the currentcollector, and the dispersion medium is vaporized. After that, theactive material layer may be pressed by a compression method such as aroll press method or a flat plate press method so as to be consolidatedif necessary.

As the dispersion medium, water, polar organic solvent such asN-methylpyrrolidone (NMP) or dimethylformamide, or the like can be used.Water is preferably used in terms of the safety and cost.

It is preferable for the binder to include, for example, water-solublepolymers. As the water-soluble polymers, a polysaccharide or the likecan be used. As the polysaccharide, a cellulose derivative such ascarboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose,hydroxypropyl cellulose, diacetyl cellulose, or regenerated cellulose,starch, or the like can be used.

As the binder, a rubber material such as styrene-butadiene rubber (SBR),styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber,butadiene rubber, fluorine rubber, or ethylene-propylene-diene copolymeris preferably used. Any of these rubber materials is more preferablyused in combination with the aforementioned water-soluble polymers.

Alternatively, as the binder, a material such as polystyrene,poly(methyl acrylate), poly(methyl methacrylate) (PMMA), sodiumpolyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO),polypropylene oxide, polyimide, polyvinyl chloride,polytetrafluoroethylene, polyethylene, polypropylene, isobutylene,polyethylene terephthalate, nylon, polyvinylidene fluoride (PVdF),polyacrylonitrile (PAN), polyvinyl chloride, ethylene-propylene-dienepolymer, polyvinyl acetate, or nitrocellulose is preferably used.

Two or more of the above materials may be used in combination for thebinder.

The content of the binder in the positive electrode active materiallayer 102 is preferably greater than or equal to 1 wt % and less than orequal to 10 wt %, more preferably greater than or equal to 2 wt % andless than or equal to 8 wt %, and still more preferably greater than orequal to 3 wt % and less than or equal to 5 wt %. The content of theconductive additive in the positive electrode active material layer 102is preferably greater than or equal to 1 wt % and less than or equal to10 wt %, more preferably greater than or equal to 1 wt % and less thanor equal to 5 wt %.

In the case where the positive electrode active material layer 102 isformed by a coating method, the positive electrode active material, thebinder, and the conductive additive are mixed to form a positiveelectrode paste (slurry), and the positive electrode paste is applied tothe positive electrode current collector 101 and dried.

2. Negative Electrode

The negative electrode 115 includes, for example, the negative electrodecurrent collector 105 and the negative electrode active material layer106 formed over the negative electrode current collector 105.

The negative electrode current collector 105 can be formed using amaterial that has high conductivity and is not alloyed with a carrierion of lithium or the like, such as stainless steel, gold, platinum,iron, copper, or titanium, or an alloy thereof. Alternatively, analuminum alloy to which an element that improves heat resistance, suchas silicon, titanium, neodymium, scandium, or molybdenum, is added canbe used. The negative electrode current collector 105 can have afoil-like shape, a plate-like shape (sheet-like shape), a net-likeshape, a punching-metal shape, an expanded-metal shape, or the like asappropriate. The negative electrode current collector 105 preferably hasa thickness greater than or equal to 5 μm and less than or equal to 30μm. The surface of the negative electrode current collector 105 may beprovided with an undercoat layer using graphite or the like.

The negative electrode current collector is preferably formed using ahigh-strength material such as stainless steel or titanium because thenegative electrode current collector can resist the change in the shapecaused by expansion of the negative electrode active material layer. Theuse of this material is particularly preferable in the case where thenegative electrode active material is a material whose volume largelychanges with charging and discharging, such as a material containingsilicon.

The negative electrode active material layer 106 may further include abinder for increasing the adhesion of negative electrode activematerials, a conductive additive for increasing the conductivity of thenegative electrode active material layer 106, and the like in additionto the negative electrode active materials. For the materials of thebinder and the conductive additive which are used for the negativeelectrode active material layer, the materials of the binder and theconductive additive which are used for the positive electrode activematerial layer can be referred to.

A material with which lithium can be dissolved and precipitated or amaterial which can reversibly react with lithium ions can be used for anegative electrode active material; for example, a lithium metal, acarbon-based material, or an alloy-based material can be used.

The lithium metal is preferable because of its low redox potential(3.045 V lower than that of a standard hydrogen electrode) and highspecific capacity per unit weight and per unit volume (3860 mAh/g and2062 mAh/cm³).

Examples of the carbon-based material include graphite, graphitizingcarbon (soft carbon), non-graphitizing carbon (hard carbon), a carbonnanotube, graphene, and carbon black.

Examples of the graphite include artificial graphite such as meso-carbonmicrobeads (MCMB), coke-based artificial graphite, or pitch-basedartificial graphite and natural graphite such as spherical naturalgraphite.

Graphite has a low potential substantially equal to that of a lithiummetal (0.1 V to 0.3 V vs. Li/Li⁺) when lithium ions are intercalatedinto the graphite (while a lithium-graphite intercalation compound isformed). For this reason, a lithium-ion power storage device can have ahigh operating voltage. In addition, graphite is preferable because ofits advantages such as relatively high capacity per unit volume, smallvolume expansion, low cost, and safety greater than that of a lithiummetal.

As the negative electrode active material, other than the above carbonmaterials, a material which enables charge-discharge reaction by analloying reaction and a dealloying reaction with carrier ions can beused. In the case where carrier ions are lithium ions, for example, amaterial containing at least one of Mg, Ca, Al, Si, Ge, Sn,

Pb, As, Sb, Bi, Ag, Au, Zn, Cd, Hg, In, etc. can be used. Such elementshave higher capacity than carbon. In particular, silicon has asignificantly high theoretical capacity of 4200 mAh/g; thus, silicon ispreferably used for the negative electrode active material. Examples ofthe material using such elements include Mg₂Si, Mg₂Ge, Mg₂Sn, SnS₂,V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb, CeSb₃,LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, and SbSn.

Alternatively, for the negative electrode active material, an oxide suchas SiO, SnO, SnO₂, titanium dioxide (TiO₂), lithium titanium oxide(Li₄Ti₅O₁₂), lithium-graphite intercalation compound (Li_(x)C₆), niobiumpentoxide (Nb₂O₅), tungsten oxide (WO₂), or molybdenum oxide (MoO₂) canbe used.

Note that SiO refers to the powder of a silicon oxide and can also bereferred to as SiO_(y) (2>y>0). SiO may include a silicon-rich portion.Examples of SiO include a material containing one or more of Si₂O₃,Si₃O₄, and Si₂O and a mixture of Si powder and silicon dioxide (SiO₂).Furthermore, SiO may contain another element (e.g., carbon, nitrogen,iron, aluminum, copper, titanium, calcium, and manganese). In otherwords, SiO refers to a colored material containing two or more of singlecrystal silicon, amorphous silicon, polycrystalline silicon, Si₂O₃,Si₃O₄, Si₂O, and SiO₂. Thus, SiO can be distinguished from SiO_(x) (x is2 or more), which is clear and colorless or white. Note that in the casewhere a power storage device is fabricated using SiO as a materialthereof and SiO is oxidized because of repeated charge and dischargecycles, SiO is changed into SiO₂ in some cases.

Still alternatively, for the negative electrode active material,Li_(3−x)M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li₂₆Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g and 1890 mAh/cm³).

A nitride containing lithium and a transition metal is preferably used,in which case lithium ions are contained in the negative electrodeactive material and thus the negative electrode active material can beused in combination with a material for a positive electrode activematerial which does not contain lithium ions, such as V₂O₅ or Cr₃O₈. Inthe case of using a material containing lithium ions as a positiveelectrode active material, the nitride containing lithium and atransition metal can be used for the negative electrode active materialby extracting the lithium ions contained in the positive electrodeactive material in advance.

Alternatively, a material that causes a conversion reaction can be usedfor the negative electrode active material. For example, a transitionmetal oxide with which an alloying reaction with lithium is not caused,such as cobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), maybe used for the negative electrode active material. Other examples ofthe material that causes a conversion reaction include oxides such asFe₂O₃, CuO, Cu₂O, RuO₂, and Cr₂O₃, sulfides such as CoS_(0.89), NiS, andCuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄, phosphides such as NiP₂,FeP₂, and CoP₃, and fluorides such as FeF₃ and BiF₃. Note that any ofthe fluorides can be used as a positive electrode active materialbecause of its high potential.

In the case where the negative electrode active material layer 106 isformed by a coating method, the negative electrode active material andthe binder are mixed to form a negative electrode paste (slurry), andthe negative electrode paste is applied to the negative electrodecurrent collector 105 and dried.

Graphene may be formed on a surface of the negative electrode activematerial layer 106. In the case of using silicon as the negativeelectrode active material, the volume of silicon greatly changes due toocclusion and release of carrier ions in charge-discharge cycles.Therefore, the adhesion between the negative electrode current collector105 and the negative electrode active material layer 106 is decreased,resulting in degradation of battery characteristics caused by charge anddischarge.

Thus, graphene is preferably formed on a surface of the negativeelectrode active material layer 106 containing silicon because even whenthe volume of silicon changes in charge-discharge cycles, decrease inthe adhesion between the negative electrode current collector 105 andthe negative electrode active material layer 106 can be inhibited, whichmakes it possible to reduce degradation of battery characteristics.

Alternatively, a coating film of an oxide or the like may be formed onthe surface of the negative electrode active material layer 106. Acoating film formed by decomposition or the like of an electrolyticsolution or the like in charging cannot release electric charges used atthe formation, and therefore forms irreversible capacity.

In contrast, the film of an oxide or the like provided on the surface ofthe negative electrode active material layer 106 in advance can reduceor prevent generation of irreversible capacity.

As the coating film coating the negative electrode active material layer106, an oxide film of any one of niobium, titanium, vanadium, tantalum,tungsten, zirconium, molybdenum, hafnium, chromium, aluminum, andsilicon or an oxide film containing any one of these elements andlithium can be used. Such a film is much denser than a conventional filmformed on a surface of a negative electrode due to a decompositionproduct of an electrolytic solution.

For example, niobium oxide (Nb₂O₅) has a low electric conductivity of10⁻⁹ S/cm and a high insulating property. For this reason, a niobiumoxide film inhibits an electrochemical decomposition reaction betweenthe negative electrode active material and the electrolytic solution. Onthe other hand, niobium oxide has a lithium diffusion coefficient of10⁻⁹ cm²/sec and high lithium ion conductivity. Therefore, niobium oxidecan transmit lithium ions. Alternatively, silicon oxide or aluminumoxide may be used.

A sol-gel method can be used to coat the negative electrode activematerial layer 106 with the coating film, for example. The sol-gelmethod is a method for forming a thin film in such a manner that asolution of metal alkoxide, a metal salt, or the like is changed into agel, which has lost its fluidity, by hydrolysis reaction andpolycondensation reaction and the gel is baked. Since a thin film isformed from a liquid phase in the sol-gel method, raw materials can bemixed uniformly on the molecular scale. Therefore, by adding a negativeelectrode active material such as graphite to a raw material of themetal oxide film which is a solvent, the active material can be easilydispersed into the gel. In such a manner, the coating film can be formedon the surface of the negative electrode active material layer 106. Adecrease in the capacity of the power storage device can be prevented byusing the coating film.

3. Separator

As a material for the separator 107, a porous insulator such ascellulose, polypropylene (PP), polyethylene (PE), polybutene, nylon,polyester, polysulfone, polyacrylonitrile, polyvinylidene fluoride,tetrafluoroethylene, or polyphenylene sulfide can be used.Alternatively, nonwoven fabric of a glass fiber or the like, or adiaphragm in which a glass fiber and a polymer fiber are mixed may beused. Alternatively, to increase heat resistance, a polyester nonwovenfabric to which ceramic is applied or which is coated with aramid may beused as a separator.

4. Electrolyte solution

As a solvent for the electrolyte solution 108 used in the power storagedevice 100, an aprotic organic solvent is preferably used. For example,one of ethylene carbonate (EC), propylene carbonate (PC), butylenecarbonate, chloroethylene carbonate, vinylene carbonate,γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethylcarbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methylacetate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane(DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile,benzonitrile, tetrahydrofuran, sulfolane, and sultone can be used, ortwo or more of these solvents can be used in an appropriate combinationin an appropriate ratio.

Alternatively, the use of one or more kinds of ionic liquids (roomtemperature molten salts) that have non-flammability and non-volatilityas the solvent for the electrolyte solution can prevent the powerstorage device from exploding or catching fire even when the powerstorage device internally shorts out or the internal temperatureincreases due to overcharging or the like.

In the case of using lithium ions as carriers, as an electrolytedissolved in the above-described solvent, one of lithium salts such asLiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiAlCl₄, LiSCN, LiBr, LiI, Li₂SO₄,Li₂B₁₀Cl₁₀, Li₂B₁₂Cl₁₂, LiCF₃SO₃, LiC₄F₉SO₃, LiC(CF₃SO₂)₃,LiC(C₂F₅SO₂)₃, LiN(FSO₂)₂, LiN(CF₃SO₂)₂, LiN(C₄F₉SO₂) (CF₃SO₂), andLiN(C₂F₅SO₂)₂ can be used, or two or more of these lithium salts can beused in an appropriate combination in an appropriate ratio.

Polymer may be added to the electrolyte solution so that the electrolytesolution becomes gelled. The electrolyte solution being gelled hashigher safety against liquid leakage or the like. Furthermore, a powerstorage device can be thinner and more lightweight. As the polymercapable of making the electrolyte solution gelled, a polyalkyleneoxide-based polymer, a polyacrylonitrile-based polymer, a polyvinylidenefluoride-based polymer, a polyacrylate-based polymer, or apolymethacrylate-based polymer can be used. Note that in thisspecification and the like, the polyvinylidene fluoride-based polymer,for example, refers to a polymer containing polyvinylidene fluoride, andincludes a poly(vinylidene fluoride-hexafluoropropylene) copolymer andthe like in its category. The formed polymer may be porous.

The above polymer can be qualitatively analyzed using a Fouriertransform infrared (FT-IR) spectrometer or the like. For example, thepolyvinylidene fluoride-based polymer has an absorption peak showing aC—F bond in a spectrum obtained by the FT-IR spectrometer. Thepolyacrylonitrile-based polymer has an absorption peak showing a C═Nbond in a spectrum obtained by the FT-IR spectrometer.

The electrolyte solution used for the power storage device is preferablya highly purified one so as to contain a negligible amount of dustparticles and elements other than the constituent elements of theelectrolyte solution (hereinafter, also simply referred to asimpurities). Specifically, the weight ratio of impurities to theelectrolyte solution is less than or equal to 1%, preferably less thanor equal to 0.1%, and more preferably less than or equal to 0.01%. Anadditive agent such as vinylene carbonate may be added to theelectrolyte solution.

Instead of the electrolyte solution, a solid electrolyte including aninorganic material such as a sulfide-based inorganic material or anoxide-based inorganic material can be used. When the solid electrolyteis used, a separator and a spacer are not necessary. Furthermore, thebattery can be entirely solidified; therefore, there is no danger ofliquid leakage, dramatically improving the safety of the battery.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 4

Described in this embodiment are examples of an electronic deviceincluding the power storage device shown in Embodiment 1.

FIG. 25 illustrates an example of an armband electronic device includinga flexible power storage device. An armband device 7300 illustrated inFIG. 25 can be worn on an arm 7301 and includes a display portion havinga curved surface and a bendable power storage device.

Note that in the display portion, a display element, a display devicewhich is a device including a display element, a light-emitting element,and a light-emitting device which is a device including a light-emittingelement can employ a variety of modes or can include a variety ofelements. The display element, the display device, the light-emittingelement, or the light-emitting device includes at least one of anelectroluminescent (EL) element (e.g., an EL element including organicand inorganic materials, an organic EL element, or an inorganic ELelement), an LED (e.g., a white LED, a red LED, a green LED, or a blueLED), a transistor (a transistor that emits light depending on current),an electron emitter, a liquid crystal element, electronic ink, anelectrophoretic element, a grating light valve (GLV), a plasma displaypanel (PDP), a display element using micro electro mechanical systems(MEMS), a digital micromirror device (DMD), a digital micro shutter(DMS), MIRASOL (registered trademark), an interferometric modulatordisplay (IMOD) element, a MEMS shutter display element, anoptical-interference-type MEMS display element, an electrowettingelement, a piezoelectric ceramic display, a display element including acarbon nanotube, and the like. In addition to that, the display element,the display device, the light-emitting element, or the light-emittingdevice may include a display medium whose contrast, luminance,reflectivity, transmittance, or the like is changed by electrical ormagnetic effect. Examples of a display device having an EL elementinclude an EL display. Examples of display devices having electronemitters include a field emission display (FED), an SED-type flat paneldisplay (SED: surface-conduction electron-emitter display), and thelike. Examples of display devices including liquid crystal elementsinclude a liquid crystal display (e.g., a transmissive liquid crystaldisplay, a transflective liquid crystal display, a reflective liquidcrystal display, a direct-view liquid crystal display, or a projectionliquid crystal display). Examples of a display device includingelectronic ink, electronic liquid powder (registered trademark), orelectrophoretic elements include electronic paper. In the case of atransflective liquid crystal display or a reflective liquid crystaldisplay, some or all of pixel electrodes function as reflectiveelectrodes. For example, some or all of pixel electrodes are formed tocontain aluminum, silver, or the like. In such a case, a memory circuitsuch as an SRAM can be provided under the reflective electrodes, leadingto lower power consumption. Note that in the case of using an LED,graphene or graphite may be provided under an electrode or a nitridesemiconductor of the LED. Graphene or graphite may be a multilayer filmin which a plurality of layers are stacked. When graphene or graphite isprovided in this manner, a nitride semiconductor, for example, an n-typeGaN semiconductor layer including crystals can be easily formedthereover. Furthermore, a p-type GaN semiconductor layer includingcrystals or the like can be provided thereover, and thus the LED can beformed. Note that an AlN layer may be provided between the n-type GaNsemiconductor layer including crystals and graphene or graphite. The GaNsemiconductor layers included in the LED may be formed by MOCVD. Notethat when the graphene is provided, the GaN semiconductor layersincluded in the LED can also be formed by a sputtering method.

Preferably, the armband device 7300 further includes one or morefunctional elements, e.g., a sensor. Examples of the sensor include asensor having a function of measuring force, displacement, position,speed, acceleration, angular velocity, rotational frequency, distance,light, liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, electric current, voltage, electric power,radiation, flow rate, humidity, gradient, oscillation, odor, or infraredrays. The armband device 7300 may include a functional element such as atouch panel, an antenna, a power generation element, or a speaker.

For example, when a user wears the armband device 7300 on his or her armand makes its display emit light at nighttime, traffic safety can beensured. For another example, when a construction crew or the like whowears a helmet wears the armband device 7300 and operates it, he or shecan exchange information by communication to easily obtain thepositional information of other crews so that he or she can work safely.

FIGS. 26A to 26F illustrate other examples of the electronic deviceincluding a flexible power storage device. Examples of electronicdevices each including a flexible power storage device includetelevision devices (also referred to as televisions or televisionreceivers), monitors of computers or the like, cameras such as digitalcameras and digital video cameras, digital photo frames, cellular phones(also referred to as mobile phones or mobile phone devices), portablegame machines, portable information terminals, audio reproducingdevices, and large game machines such as pachinko machines.

In addition, a flexible power storage device can be incorporated along acurved inside/outside wall surface of a house or a building or a curvedinterior/exterior surface of a car.

FIG. 26A illustrates an example of a cellular phone. A cellular phone7400 includes a display portion 7402 incorporated in a housing 7401,operation buttons 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the cellular phone 7400includes a power storage device 7407.

FIG. 26B illustrates the cellular phone 7400 that is bent. When thewhole cellular phone 7400 is bent by external force, the power storagedevice 7407 included in the cellular phone 7400 is also bent. FIG. 26Cillustrates the bent power storage device 7407. The power storage device7407 is a thin power storage device. The power storage device 7407 isfixed while being bent. Note that the power storage device 7407 includesa lead electrode 7408 electrically connected to a current collector7409. The current collector 7409 is, for example, copper foil, andpartly alloyed with gallium so as to improve the adhesion between thecurrent collector 7409 and an active material layer in contact with thecurrent collector 7409. Consequently, the power storage device 7407 canhave high reliability even in a state of being bent.

FIG. 26D illustrates an example of a bangle display device. A portabledisplay device 7100 includes a housing 7101, a display portion 7102,operation buttons 7103, and a power storage device 7104. FIG. 26Eillustrates the bent power storage device 7104. When the display deviceis worn on a user's arm while the power storage device 7104 is bent, theshape of the housing changes to change the curvature of part or thewhole of the power storage device 7104. Note that the radius ofcurvature of a curve at a point refers to the radius of the circular arcthat best approximates the curve at that point. The reciprocal of theradius of curvature is curvature. Specifically, part or the whole of thehousing or the main surface of the power storage device 7104 is changedin the range of radius of curvature from 40 mm to 150 mm. When theradius of curvature at the main surface of the power storage device 7104is 40 mm to 150 mm, the reliability can be kept high.

Furthermore, the flexile power storage device which can be bent byexternal force can be provided with high space efficiency in any of avariety of electronic devices. For example, in a stove 7500 illustratedin FIG. 26F, a module 7511 is attached to a main body 7512. The module7511 includes a power storage device 7501, a motor, a fan, an air outlet7511 a, and a thermoelectric generation device. In the stove 7500, aftera fuel is injected through an opening 7512 a and ignited, outside aircan be sent through the air outlet 7511 a to the inside of the stove7500 by rotating the motor and the fan which are included in the module7511 using power of the power storage device 7501. In this manner, thestove 7500 can have strong heating power because outside air can betaken into the inside of the stove 7500 efficiently. In addition,cooking can be performed on an upper grill 7513 with thermal energygenerated by the combustion of fuel. When the thermal energy isconverted into power with the thermoelectric generation device of themodule 7511, the power storage device 7501 can be charged with thepower. The power charged into the power storage device 7501 can beoutput through an external terminal 7511 b.

The power storage device described in Embodiment 1 can be provided inwearable devices illustrated in FIGS. 27A to 27C.

For example, the power storage device can be provided in a glasses-typedevice 400 illustrated in FIG. 27A. The glasses-type device 400 includesa frame 400 a and a display portion 400 b. The power storage device isprovided in a temple of the frame 400 a having a curved shape, wherebythe glasses-type device 400 can have a well-balanced weight and can beused continuously for a long time.

The power storage device can also be provided in a headset-type device401.

The headset-type device 401 includes at least a microphone portion 401a, a flexible pipe 401 b, and an earphone portion 401 c. The powerstorage device can be provided in the flexible pipe 401 b and theearphone portion 401 c.

Furthermore, the power storage device can be provided in a device 402that can be attached directly to a body. A power storage device 402 bcan be provided in a thin housing 402 a of the device 402.

Furthermore, the power storage device can be provided in a device 403that can be attached to clothes. A power storage device 403 b can beprovided in a thin housing 403 a of the device 403.

Furthermore, the power storage device can be provided in a watch-typedevice 405. The watch-type device 405 includes a display portion 405 aand a belt portion 405 b, and the power storage device can be providedin the display portion 405 a or the belt portion 405 b.

FIG. 27B is an enlarged view of the watch-type device 405. In thewatch-type device 405, a power storage device 405 c is provided in thebent, round display portion 405 a. For the display portion that can beused for the display portion 405 a, the description of the displayportion in FIG. 25 can be referred to. The display portion 405 a candisplay various kinds of information such as time and receptioninformation of an e-mail or an incoming call. The outer periphery of thedisplay portion 405 a of the watch-type device 405 may be a distortedclosed curve as illustrated in FIG. 27C. The power storage device 405 ccan be provided along the display portion 405 a.

Furthermore, the power storage device can be provided in a belt-typedevice 406. The belt-type device 406 includes a belt portion 406 a and awireless power feeding and receiving portion 406 b, and the powerstorage device can be provided inside the belt portion 406 a.

In addition, the watch-type device 405 is a wearable device that iswound around an arm directly; thus, a sensor that measures the pulse,the blood pressure, or the like of the user may be incorporated therein.Data on the exercise quantity and health of the user can be stored to beused for health maintenance.

Furthermore, devices that can be carried around, such as theabove-described armband device 7300, cellular phone 7400, portabledisplay device 7100, belt-type device 406, and watch-type device 405,may be provided with a positioning system such as the global positioningsystem (GPS). With the system, the user can find his/her presentposition, and the system is useful in dealing with kidnapping,wandering, and the like.

The watch-type device will be described in more detail with reference toFIGS. 28A to 29B.

A watch-type device 500 illustrated in FIGS. 28A and 28B includes ahousing 501, a power storage device 502, a display portion 503, a logicboard 504, an antenna 505, a sensor 506, a microphone 507, a speaker508, and a belt portion 509. FIG. 28A is a front view of the powerstorage device 502. FIG. 28B is a cross-sectional view of the watch-typedevice 500 along dashed-dotted line AB in FIG. 28A. When the housing501, the display portion 503, and the logic board 504 have flexibility,the watch-type device can be curved along a user's arm.

The watch-type device 500 illustrated in FIGS. 29A and 29B includes thehousing 501, the power storage device 502, the display portion 503, thelogic board 504, the antenna 505, the sensor 506, a solar battery 510,operation buttons 511, and the belt portion 509. FIG. 29A is a frontview of the power storage device 502. FIG. 29B is a cross-sectional viewof the watch-type device 500 along dashed-dotted line AB in FIG. 29A.The power storage device 502 can be charged by, for example, generatingpower in the solar battery 510.

FIG. 30 illustrates other examples of electronic devices. In FIG. 30, adisplay device 8000 is an example of an electronic device including apower storage device 8004 of one embodiment of the present invention.Specifically, the display device 8000 corresponds to a display devicefor TV broadcast reception and includes a housing 8001, a displayportion 8002, speaker portions 8003, the power storage device 8004, andthe like. The power storage device 8004 of one embodiment of the presentinvention is provided in the housing 8001. The display device 8000 canreceive electric power from a commercial power supply. Alternatively,the display device 8000 can use electric power stored in the powerstorage device 8004. Thus, the display device 8000 can operate with theuse of the power storage device 8004 of one embodiment of the presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoresis displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 8002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like besides TV broadcast reception.

In FIG. 30, an installation lighting device 8100 is an example of anelectronic device including a power storage device 8103 of oneembodiment of the present invention. Specifically, the lighting device8100 includes a housing 8101, a light source 8102, the power storagedevice 8103, and the like. Although FIG. 30 illustrates the case wherethe power storage device 8103 is provided in a ceiling 8104 on which thehousing 8101 and the light source 8102 are installed, the power storagedevice 8103 may be provided in the housing 8101. The lighting device8100 can receive electric power from a commercial power supply.Alternatively, the lighting device 8100 can use electric power stored inthe power storage device 8103. Thus, the lighting device 8100 canoperate with the use of the power storage device 8103 of one embodimentof the present invention as an uninterruptible power supply even whenelectric power cannot be supplied from a commercial power supply due topower failure or the like.

Note that although the installation lighting device 8100 provided in theceiling 8104 is illustrated in FIG. 30 as an example, the power storagedevice of one embodiment of the present invention can be used in aninstallation lighting device provided in, for example, a wall 8105, afloor 8106, or a window 8107 other than the ceiling 8104. Alternatively,the power storage device of one embodiment of the present invention canbe used in a tabletop lighting device or the like.

As the light source 8102, an artificial light source which emits lightartificially by using electric power can be used. Specifically, anincandescent lamp, a discharge lamp such as a fluorescent lamp, andlight-emitting elements such as an LED and an organic EL element aregiven as examples of the artificial light source.

In FIG. 30, an air conditioner including an indoor unit 8200 and anoutdoor unit 8204 is an example of an electronic device including apower storage device 8203 of one embodiment of the present invention.Specifically, the indoor unit 8200 includes a housing 8201, an airoutlet 8202, the power storage device 8203, and the like. Although FIG.30 illustrates the case where the power storage device 8203 is providedin the indoor unit 8200, the power storage device 8203 may be providedin the outdoor unit 8204. Alternatively, the power storage devices 8203may be provided in both the indoor unit 8200 and the outdoor unit 8204.The air conditioner can receive electric power from a commercial powersupply. Alternatively, the air conditioner can use electric power storedin the power storage device 8203. Particularly in the case where thepower storage devices 8203 are provided in both the indoor unit 8200 andthe outdoor unit 8204, the air conditioner can operate with the use ofthe power storage device 8203 of one embodiment of the present inventionas an uninterruptible power supply even when electric power cannot besupplied from a commercial power supply due to power failure or thelike.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 30 as an example, thepower storage device of one embodiment of the present invention can beused in an air conditioner in which the functions of an indoor unit andan outdoor unit are integrated in one housing.

In FIG. 30, an electric refrigerator-freezer 8300 is an example of anelectronic device including a power storage device 8304 of oneembodiment of the present invention. Specifically, the electricrefrigerator-freezer 8300 includes a housing 8301, a refrigerator door8302, a freezer door 8303, the power storage device 8304, and the like.The power storage device 8304 is provided in the housing 8301 in FIG.30. The electric refrigerator-freezer 8300 can receive electric powerfrom a commercial power supply. Alternatively, the electricrefrigerator-freezer 8300 can use electric power stored in the powerstorage device 8304. Thus, the electric refrigerator-freezer 8300 canoperate with the use of the power storage device 8304 of one embodimentof the present invention as an uninterruptible power supply even whenelectric power cannot be supplied from a commercial power supply due topower failure or the like.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 5

Described in this embodiment are examples of vehicles including thepower storage device shown in Embodiment 1.

The use of the power storage device in vehicles can lead tonext-generation clean energy vehicles such as hybrid electric vehicles(HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles(PHEVs).

FIGS. 31A and 31B each illustrate an example of a vehicle using oneembodiment of the present invention. An automobile 8400 illustrated inFIG. 31A is an electric vehicle which runs on the power of the electricmotor. Alternatively, the automobile 8400 is a hybrid electric vehiclecapable of driving using either the electric motor or the engine asappropriate. One embodiment of the present invention achieves ahigh-mileage vehicle. The automobile 8400 includes the power storagedevice. The power storage device is used not only for driving theelectric motor, but also for supplying electric power to alight-emitting device such as a headlight 8401 or a room light (notillustrated).

The power storage device can also supply electric power to a displaydevice of a speedometer, a tachometer, or the like included in theautomobile 8400. Furthermore, the power storage device can supplyelectric power to a semiconductor device included in the automobile8400, such as a navigation system.

FIG. 31B illustrates an automobile 8500 including the power storagedevice. The automobile 8500 can be charged when the power storage deviceis supplied with electric power through external charging equipment by aplug-in system, a contactless power feeding system, or the like. In FIG.31B, a power storage device included in the automobile 8500 is chargedwith the use of a ground-based charging apparatus 8021 through a cable8022. In charging, a given method such as CHAdeMO (registered trademark)or Combined Charging System may be employed as a charging method, thestandard of a connector, or the like as appropriate. The chargingapparatus 8021 may be a charging station provided in a commerce facilityor a power source in a house. For example, with the use of a plug-intechnique, the power storage device included in the automobile 8500 canbe charged by being supplied with electric power from outside. Thecharging can be performed by converting AC electric power into DCelectric power through a converter such as an AC-DC converter.

Further, although not illustrated, the vehicle may include a powerreceiving device so as to be charged by being supplied with electricpower from an above-ground power transmitting device in a contactlessmanner. In the case of the contactless power feeding system, by fittinga power transmitting device in a road or an exterior wall, charging canbe performed not only when the electric vehicle is stopped but also whendriven. In addition, the contactless power feeding system may beutilized to perform transmission and reception of electric power betweenvehicles. Furthermore, a solar battery may be provided in the exteriorof the automobile to charge the power storage device when the automobilestops or moves. To supply electric power in such a contactless manner,an electromagnetic induction method or a magnetic resonance method canbe used.

Furthermore, the power storage device included in the vehicle can beused as a power source for supplying electric power to products otherthan the vehicle. In such a case, the use of a commercial power sourcecan be avoided at peak time of electric power demand.

An example of a motorcycle using one embodiment of the present inventionwill be described with reference to FIGS. 32A and 32B.

A motor scooter 8600 illustrated in FIG. 32A includes a power storagedevice 8602 in a side mirror 8601. The power storage device 8602 can bebent and therefore can be provided with high space efficiency in theside mirror 8601 even with a curved shape. FIG. 32B is an enlarged viewof the side mirror 8601 seen from the front of the motor scooter 8600.The side mirror 8601 includes an indicator 8603. The power storagedevice 8602 can supply electric power to the indicator 8603.

Furthermore, in the motor scooter 8600 illustrated in FIG. 32A, thepower storage device 8602 can be held in a storage unit under seat 8604.The power storage device 8602 can be bent and therefore can be held inthe storage unit under seat 8604 even with a small size by being bent orfolded.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

This application is based on Japanese Patent Application serial No.2015-088095 filed with Japan Patent Office on Apr. 23, 2015, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A power storage device comprising: a positiveelectrode comprising a positive electrode current collector and apositive electrode active material layer in contact with the positiveelectrode current collector; a negative electrode comprising a negativeelectrode current collector and a negative electrode active materiallayer in contact with the negative electrode current collector; anexterior body; and an electrolyte, wherein the positive electrode activematerial layer and the negative electrode active material layer overlapwith each other, wherein an outer periphery of each of the positiveelectrode active material layer and the negative electrode activematerial layer is a first closed curve, wherein the exterior bodyincludes a film and a thermocompression-bonded region, wherein an innerperiphery of the thermocompression-bonded region is a second closedcurve, wherein the first closed curve and the second closed curveinclude no vertex, and wherein the electrolyte, the positive electrodeactive material layer, and the negative electrode active material layerare in a region surrounded by the thermocompression-bonded region. 2.The power storage device according to claim 1, wherein the outerperiphery of each of the positive electrode active material layer andthe negative electrode active material layer is approximately circular,and wherein the inner periphery of the thermocompression-bonded regionis approximately circular.
 3. The power storage device according toclaim 1, wherein the film includes a projection or a depression.
 4. Thepower storage device according to claim 3, wherein an inner or outerperiphery of the projection or the depression has a shape similar tothat of the outer periphery of the positive electrode active materiallayer or the negative electrode active material layer.
 5. The powerstorage device according to claim 1, wherein the power storage devicehas flexibility.
 6. An electronic device comprising: the power storagedevice according to claim 1; and a housing having flexibility.
 7. Anelectronic device comprising: the power storage device according toclaim 1; and a housing having a curved portion.
 8. A power storagedevice comprising: a positive electrode comprising a positive electrodecurrent collector and a positive electrode active material layer incontact with the positive electrode current collector; a negativeelectrode comprising a negative electrode current collector and anegative electrode active material layer in contact with the negativeelectrode current collector; an exterior body; and an electrolyte,wherein the positive electrode active material layer and the negativeelectrode active material layer overlap with each other, wherein anouter periphery of each of the positive electrode active material layerand the negative electrode active material layer is a first closedcurve, wherein the exterior body includes a film, wherein the exteriorbody includes a first thermocompression-bonded region and a secondthermocompression-bonded region, wherein the firstthermocompression-bonded region is surrounded by the secondthermocompression-bonded region, wherein an outer periphery of the firstthermocompression-bonded region is a second closed curve, wherein aninner periphery of the second thermocompression-bonded region is a thirdclosed curve, wherein the first closed curve and the second closed curveinclude no vertex, wherein the exterior body includes an opening in aregion surrounded by the first thermocompression-bonded region, whereinthe electrolyte, the positive electrode active material layer, and thenegative electrode active material layer are in a region between thefirst thermocompression-bonded region and the secondthermocompression-bonded region, wherein the positive electrode currentcollector includes a portion extending in the opening, and wherein thenegative electrode current collector includes a portion extending in theopening.
 9. The power storage device according to claim 8, wherein theouter periphery of each of the positive electrode active material layerand the negative electrode active material layer is approximatelycircular, and wherein the inner periphery of the secondthermocompression-bonded region is approximately circular.
 10. The powerstorage device according to claim 8, wherein the film includes aprojection or a depression.
 11. The power storage device according toclaim 10, wherein an inner or outer periphery of the projection or thedepression has a shape similar to that of the outer periphery of thepositive electrode active material layer or the negative electrodeactive material layer.
 12. The power storage device according to claim8, wherein the power storage device has flexibility.
 13. An electronicdevice comprising: the power storage device according to claim 8; and ahousing having flexibility.
 14. An electronic device comprising: thepower storage device according to claim 8; and a housing having a curvedportion.
 15. A power storage device comprising: a positive electrodecomprising a positive electrode current collector and a positiveelectrode active material layer in contact with the positive electrodecurrent collector; a negative electrode comprising a negative electrodecurrent collector and a negative electrode active material layer incontact with the negative electrode current collector; an exterior body;and an electrolyte, wherein the positive electrode active material layerand the negative electrode active material layer overlap with eachother, wherein an outer periphery of each of the positive electrodeactive material layer and the negative electrode active material layeris a first closed curve, wherein the exterior body includes a film,wherein the exterior body includes a first thermocompression-bondedregion and a second thermocompression-bonded region, wherein the firstthermocompression-bonded region is surrounded by the secondthermocompression-bonded region, wherein an outer periphery of the firstthermocompression-bonded region is a second closed curve, wherein aninner periphery of the second thermocompression-bonded region is a thirdclosed curve, wherein the first closed curve and the second closed curveinclude no vertex, wherein the exterior body includes an opening in aregion surrounded by the first thermocompression-bonded region, whereinthe electrolyte, the positive electrode active material layer, and thenegative electrode active material layer are in a region between thefirst thermocompression-bonded region and the secondthermocompression-bonded region, wherein the positive electrode lead iselectrically connected to the positive electrode current collector inthe region between the first thermocompression-bonded region and thesecond thermocompression-bonded region, wherein the positive electrodelead includes a portion extending in the opening, wherein the negativeelectrode lead is electrically connected to the negative electrodecurrent collector in the region between the firstthermocompression-bonded region and the second thermocompression-bondedregion, and wherein the negative electrode lead includes a portionextending in the opening.
 16. The power storage device according toclaim 15, wherein the outer periphery of each of the positive electrodeactive material layer and the negative electrode active material layeris approximately circular, and wherein the inner periphery of the secondthermocompression-bonded region is approximately circular.
 17. The powerstorage device according to claim 15, wherein the film includes aprojection or a depression.
 18. The power storage device according toclaim 17, wherein an inner or outer periphery of the projection or thedepression has a shape similar to that of the outer periphery of thepositive electrode active material layer or the negative electrodeactive material layer.
 19. The power storage device according to claim15, wherein the power storage device has flexibility.
 20. An electronicdevice comprising: the power storage device according to claim 15; and ahousing having flexibility.
 21. An electronic device comprising: thepower storage device according to claim 15; and a housing having acurved portion.
 22. A power storage device comprising: a positiveelectrode comprising a positive electrode current collector and apositive electrode active material layer in contact with the positiveelectrode current collector; a negative electrode comprising a negativeelectrode current collector and a negative electrode active materiallayer in contact with the negative electrode current collector; anexterior body; and an electrolyte, wherein the positive electrode activematerial layer and the negative electrode active material layer overlapwith each other, wherein an outer periphery of each of the positiveelectrode active material layer and the negative electrode activematerial layer is a first closed curve, wherein the exterior bodyincludes a film and a thermocompression-bonded region, wherein an innerperiphery of the thermocompression-bonded region is a second closedcurve, wherein the first closed curve and the second closed curve haveno singular point, and wherein the electrolyte, the positive electrodeactive material layer, and the negative electrode active material layerare in a region surrounded by the thermocompression-bonded region. 23.The power storage device according to claim 22, wherein the filmincludes a projection or a depression.
 24. The power storage deviceaccording to claim 22, wherein the power storage device has flexibility.25. An electronic device comprising: the power storage device accordingto claim 22; and a housing having flexibility.
 26. An electronic devicecomprising: the power storage device according to claim 22; and ahousing having a curved portion.